Towards the Necessary Decarbonization of Historic Buildings: A Review

Abstract

The critical and urgent issue of decarbonization by 2050 needs to include the existing historical built environment in the process of energy requalification. These buildings, subjected to heritage preservation, are extremely inadequate to the modern standards of energy efficiency and thermal comfort, and they exhibit the poorest energy performance. In this study, a review of the existing scientific literature on the matter of energy renovation processes applied to historic buildings is provided. The reviewed papers, selected from scientific databases, were initially categorized according to their reference scale—either individual buildings or urban contexts. Subsequently, the papers were grouped on the basis of the main energy efficiency levels they investigated. The goal is to offer a comprehensive overview of the materials, technologies and strategies currently in use, as well as future perspectives, to aid the ecological transition and foster sustainable development, all while preserving the artistic, cultural and architectural heritage of these buildings.

1. Introduction

In recent years, the energy issue has become increasingly relevant on both the local and global scales. The ongoing dependence on fossil fuels has raised concerns about greenhouse gas emissions, which significantly contribute to climate change.

In Europe, buildings are responsible for 40% of energy consumption and 36% of both direct and indirect energy-related greenhouse gas emissions [1]. For this reason, the building sector plays a crucial role in adopting technologies aimed at reducing energy waste and boosting sustainable development.

Over time, the European Union has introduced numerous directives and regulations to promote energy efficiency in buildings, with the aim of reducing the emissions of pollutants (CO₂, NO_x, PM etc.) and decreasing the use of fossil fuels in energy conversion by fostering the adoption of renewable energy sources.

The new Energy Performance of Buildings Directive (EPBD 2024/1275) [2], approved by the European Union, is a cornerstone of the “Fit for 55” package. The directive mandates that by 2030, all new buildings must be zero-emission, with an earlier deadline for public buildings, for which this obligation will come into force in 2028. The long-term goal is to bring the net emissions of all of the European Union’s building stock to zero by 2050. These objectives will require a comprehensive and intensive process of refurbishment of the built environment.

Some properties, such as historic or architecturally significant buildings, as well as those located in protected areas, will be exempt from these provisions if the energy refurbishment negatively affects the heritage and cultural peculiarities. According to article 5, paragraph 2 of EPBD 2024/1275, each member state, after the establishment of the minimum energy performance requirements (as provided by paragraph 1 of Article 5), could adapt such requirements to the protected and historical buildings with the constrain to preserve their historical, architectural and artistic aspects. Therefore, the energy retrofit is not mandatory, but each member state can legislate in the matter of the energy efficiency of historic buildings. However, it is not possible to completely ignore such types of buildings, especially in the European built environment, which houses about half of the sites included in the UNESCO World Heritage list [3]. Italy, in particular, hosts the highest number of these sites [4], without considering the large number of buildings located in historic Italian centers. These buildings are notoriously energy inefficient due to traditional construction techniques and poor insulation, and moreover, are not adequate at guaranteeing acceptable comfort conditions, representing a major challenge. For these reasons, significant attention has been posed on this sector by the European Commission and the International Energy Agency [5], with the aim of promoting cultural heritage within the framework of a sustainable Europe.

While energy efficiency upgrades are necessary, the interventions must be carried out with great care to avoid compromising the cultural and architectural value of the buildings. It is essential to adopt tailored, non-invasive and reversible solutions that reduce energy demand and improve indoor environmental conditions without altering the historical integrity of the buildings, guaranteeing a balance between heritage preservation and environmental sustainability.

The aim of the presented study is to examine and analyze the scientific literature on the matter of energy renovation applied to historic buildings, highlighting the benefits, limitations and potentialities for the future. Our intention is to provide a clear overview of the current adopted materials, technologies and prospects to support the ecological transition and promote sustainable development while preserving the cultural and architectural value of these buildings.

2. Materials and Methods

The articles reviewed are the result of an in-depth study in the field of the energy retrofitting of historic buildings.

In the present study, the method adopted to select the papers is based on the following steps:

• Research papers in the scientific database “Scopus” containing the specific keywords “historic buildings” or “cultural heritage”, coupled with one or more of the following: “energy”, “retrofit”, “envelope”, “systems”, “renewable energy sources”, “thermal comfort”, “economic analysis” and “urban scale”, were identified.
• More than 80 papers were found, and after an initial check, 46 papers were selected and deeply examined to identify the similarities and differences in the proposed energy retrofit measures, the type of investigation, the intended use of the building and the climate location.
• To better understand the variety of possible interventions and their related impacts, a taxonomy that organizes and classifies the different energy retrofitting strategies into distinct categories was adopted. This taxonomic approach allows for a systematic breakdown of the interventions, facilitating the analysis of applicable solutions based on the specific characteristics of the buildings and the available technologies.

The collection of these data is needed to propose a classification of the studies and organize the present review. What emerged is a majority of work investigating a single building, about 40 papers, in comparison to papers investigating an urban district, only 6 papers.

The review study presented here is therefore organized into the following sections:

• Section 3. Building scale: Energy retrofit of historic buildings; and
• Section 4. Urban scale: Energy retrofit of historic buildings. This section explores energy retrofitting extended to entire neighborhoods or cities. The studies in this part focus on the large-scale effects of retrofitting interventions, evaluating the overall impact at the urban level.

Concerning Section 3, several subsections can be identified according to the investigated energy retrofit measures:

3.1. Interventions on the building envelope. This section examines studies focused on the passive retrofit of buildings. It analyzes interventions such as the application of insulating materials, the replacement of windows with more energy-efficient models, and improvements in building airtightness, aiming to reduce energy demand.

3.2. Interventions on building systems. Here, the focus shifts to improving the energy efficiency of existing systems by replacing them with more technologically advanced solutions. This strategy is especially adopted in contexts where it is not possible to intervene on the building envelope, and the implementation of renewable energy sources is challenging.

3.3. Combination of passive retrofit strategies and system efficiency improvements. In this section, passive and active retrofitting strategies are combined to assess their overall impact and the potential benefits resulting from their synergy.

3.4. Integration of renewable energy sources in energy retrofitting plans. This section focuses on studies investigating the introduction of energy systems based on renewable sources. It addresses the challenge of integrating these technologies into historic buildings, including studies that combine different strategies to achieve a deep and effective retrofit.

Finally, in Section 5, a discussion about the common points and differences among the reviewed studies is presented to identify some gaps for future studies in energy improvement for historical buildings and to identify/suggest aspects to focus the attention on.

A flowchart concerning the organization of the manuscript is provided in Figure 1.

3. Building Scale: Energy Retrofit of Historic Buildings

3.1. Building Envelope Interventions

When an existing building undergoes a process of energy refurbishment, the most common interventions primarily focus on the building envelope. Specifically, solutions such as the application of insulating materials and the replacement of windows with more efficient models are adopted, along with improvements in airtightness to reduce air infiltration. This approach is also common for the historic buildings sector, even if it must comply with specific heritage preservation constraints. Indeed, for this category of buildings, actions on the external façades are allowed only if they do not alter the architectural, technical and aesthetic integrity of the building, while those on the roofs must minimize visual impact: this is the case, for instance, of solar systems.

In this section, the main findings achieved thanks to the energy refurbishment of the building envelope are collected and compared.

A historic building of 1921, used as a conservatory in Valencia, is investigated in [6], proposing measures on the walls, roof and window components to meet the minimum standards required for passive building classification. The reduction in the thermal transmittance values of about 80–85% for the roof, floor and vertical walls, coupled with the replacement of transparent and frame elements to achieve a better air tightness, resulted in a 55.2% reduction in winter energy demand. However, these solutions led to an increase in cooling demand, making additional actions, such as increased natural ventilation, necessary.

A similar retrofit intervention is proposed by Ascione et al. [7] for a historical building in Benevento (whose construction began in 1927), in climatic zone C in South Italy. In this case, the replacement of windows with low-emissivity models, the internal insulation of the walls with thermal plaster and the insulation of the roof slab with expanded polystyrene panels involved annual primary energy savings of about 38%, with reductions in heating and cooling energy demand of about 49% and 12%, respectively. Therefore, a higher effectiveness is recognized in winter, but there is no increase in summer, as in Valencia’s climate [6]. In this study, the feasibility of the interventions was also analyzed, achieving a payback period of around 15 years (by considering a discount rate and an increment in electricity cost of around 3%), a fully acceptable value considering the longevity and sustainability of the adopted measures.

The energy efficiency of building envelope renovation is closely linked to the location where the measures are applied, as different climatic conditions affect the specific energy needs of each area. In this context, Galatioto et al. [8] conducted thermodynamic simulations on a model of a historic office building in four distinct climatic zones: Cagliari (Zone C), Rome (Zone D), Palermo (Zone B) and Milan (Zone E). Significant differences in terms of the impact on space heating and cooling according to the proposed interventions and the climatic conditions are achieved. The intervention on the transparent envelope, involving window replacement and the adoption of shading systems, led to an increase in heating consumption in climate zones B and C. This is mainly due to a non-correct design and the selection of window components according to exposure, as well as to the adoption of fixed shading systems, involving a reduction in the solar heat gain, which, instead, is a positive effect in the winter season and should be maximized in order to exploit the free gain provided by the outdoor environment. Conversely, the thermal insulation of opaque components, the walls (from the interior side) and roof, was highly effective in the winter season in all climate zones but resulted in an increased cooling energy demand. Nonetheless, the positive effect in winter is higher than the negative effect in the summer season in all locations. Therefore, a correct balance and a correct selection and combination of technologies should encompass other aspects, such as the indoor thermal comfort and the running costs compared to the investment ones.

Globally, from an annual perspective, the most energy-effective intervention involved improving both the opaque and transparent envelope components, with savings of about 51% in Palermo, 52% in Cagliari, 56% in Rome and 43% in Milan. From an environmental perspective, this scenario also proved to be the best, as it allowed for the highest reductions in CO₂ emissions compared to other scenarios across all areas of interest. However, these promising interventions present a payback time (PBT) of over 30 years, complicating their large-scale adoption. To overcome this issue, the authors proposed the integration of photovoltaic (PV) systems with the building retrofit measures, achieving a positive economic balance. However, it should be noted that the installation of these technologies often faces regulatory restrictions related to heritage preservation, as in the presented case study, and therefore, are not practicable. In these cases, only economic incentive policies can play a crucial role in facilitating the effective energy retrofitting of historic buildings.

The external façades of historical buildings are one of the main challenges in the energy retrofitting project because of the existing restrictions that are in place to preserve the value of the building, which, therefore, limits the adoption of external thermal insulation practices. In such cases, internal wall insulation becomes a necessary solution to reduce winter energy demand. However, this option also leads to a reduction in the available interior space.

To address the issue of the reduction in the available indoor environments, alternative materials, such as Vacuum Insulating Panels (VIPs), are proposed and investigated in the literature, as successively reported. These materials are characterized by a higher thermal performance compared to the traditional insulating materials, guaranteeing to achieve good performance with thinner layers, which means a reduction in the loss of available indoor spaces. These materials are usually characterized by a higher investment cost, which is balanced by their higher performance, but an accurate analysis is required in any case.

Qu et al. [9] proposed the energy efficiency renovation of a Victorian house in Nottingham. The main difference with previous studies is that they investigated several options for both the replacement of windows with more efficient solutions and the improvement in the building’s airtightness—such as vacuum glazing, double glazing with low-E coating, triple glazing, double vacuum glazing, double vacuum glazing with low-E coating and different types of materials for the internal wall insulation. The tested materials include the following: vacuum insulation, silica and starch aerogel, polyisocyanurate (PIR) and mineral wool, with thicknesses of 2, 4, and 8 cm, corresponding to the volume reductions of 1.3%, 2.6%, and 5.2%, respectively. To avoid a significant loss of internal space, the researchers recommended limiting the volume reduction to 1.3%, which clearly limits the energetic effect of the solution.

The cost-optimal solution consisted of using vacuum-insulated windows, reducing air infiltration with plaster and a layer of PIR of 2 cm thick. This combination leads to a 51.8% reduction in primary energy demand, with an initial investment cost of around 134 €/m² and a discounted payback period of 18 years. Besides the energy and economic analysis, they proposed an investigation concerning the thermal comfort of the occupants. Thermal comfort refers to a condition in which an occupant is in a satisfactory condition with respect to the indoor microclimatic environment. It is influenced by a combination of parameters, both objective and subjective, and the achievement of an optimal thermal comfort is necessary for the well-being of occupants and to properly address different activities inside the environments. In the investigation proposed in [9], winter indoor comfort results greatly improved, but at the same time, there was an increase of 2.6 °C in the summer indoor air temperature compared to the standard temperature of 28 °C, so appropriate ventilation systems are recommended. More stable indoor air temperature and relative humidity conditions were also achieved in a university office building in Seoul with the addition of a second window [10].

The study conducted by Lucchi et al. [11] on a historic building near Lake Como, in climatic zone E in Italy, also addresses the evaluation of various materials for the internal wall insulation, relying on the “cost-optimal” method. In this case, the analysis examined organic materials (mineral wood fiber, flexible wood fiber, cork, XPS and EPS) and inorganic materials (perlite, microporous calcium silicate, mineral wool, glass wool and foam glass) with thicknesses of 10 and 20 cm. As expected, the economic performance index is lower for the option with a greater thickness. The evaluation of thermal efficiency in relation to cost revealed that both organic and inorganic materials offer good performance. In general, the most cost-effective materials are mineral glass wool, flexible and mineral wood fibers, XPS and EPS, thanks to their thermal conductivity values of between 0.034 and 0.038 W/mK and moderate costs. Specifically, the life cycle cost analysis showed that, with a 0.20 m insulation thickness, materials such as EPS, XPS, and wood fibers (both mineral and flexible) have the lowest payback time of less than 5 years. With a thickness of 0.10 m, EPS, XPS and mineral glass wool proved to be the most economical, with a PBT of less than 5 years. Ultimately, mineral glass wool was the most cost-effective due to its low cost and good energy performance (λ = 0.032 W/mK).

Materials such as cork, perlite and foam glass, although more expensive compared to conventional options, offer significant energy-efficient benefits. Beyond their thermal insulating properties, these materials also provide better hygrothermal management. This means that they not only help to regulate temperature but also control moisture levels within building structures, reducing the risk of condensation, mold and deterioration. The analysis demonstrated that thermal insulation reduces the heating demand and drastically lowers the thermal transmittance of the walls, bringing the building closer to Nearly Zero Energy Building (nZEB) standards. Moreover, the non-insulating case (where no intervention is carried out) is always the worst, both from an energy and economic point of view, especially in a long-term analysis, demonstrating the benefits of the proposed interventions. An analysis of the thickness of the insulating materials is also conducted in [12] for a typical historical building located in Portugal. The study was conducted for different localities, characterized by different heating degree days (HDD), and the results highlighted the need for a tailored approach, as the optimal thickness to reduce the energy demand changes by locality. The authors also emphasized that, without altering the historical value of the building, it is possible to reach a significant improvement in the energy and environmental performance of the building.

The topic on hygrothermal management emerged in [11], and therefore, the risk of condensation has to be carefully considered. In Figure 2A, a typical tuff wall is shown. It can be seen that even though the thermal transmittance is quite high (a certain minimal resistance is due to the thickness of the wall), from the point of view of condensation risk, the wall is safe. By insulating with a traditional material, on the inner side (the only chance, often, for ancient buildings as emerged from the previous analyzed studies) the condensation risk is quite high (Figure 2B), and this is due to the reduction in the temperature of the wall, implying a reduction in the vapor saturation pressure. By using thermal plaster, the thermal transmittance reduction is lower and the condensation risk is also lower (Figure 2C). Nonetheless, when the insulation intervention is applied from the inner side, paying very high care to the hygrometric behavior is necessary, as selecting suitable materials also concerns the permeability of the wall, besides the conductivity.

The thermal insulation of vertical walls, when working with historical buildings, is extremely critical, as previously discussed, but the issue of external application is not the only one. Indeed, if, on the one hand, external interventions are often restricted to preserve the artistic and historical value of a building, it is also possible to encounter cases where internal constraints must be addressed. Applying thermal insulation to the interior side of the vertical walls can be prohibited due to the presence of valuable frescoes.

For this reason, in the case of Villa Mondragone in Roma, in climatic zone D in Italy [13], the possibility of intervening on the already damaged external walls, which require restoration, was considered. Three scenarios were compared: the baseline scenario, which involves a partial use of the building (4000 m²), and two renovation scenarios. The first scenario involves replacing the window components on the first floor and on a part of the second floor of the villa, using double glazing and wooden frames. The second scenario includes insulating the external walls with diathonite (Diasen^®, Sassoferrato, Italy) and insulating the floors above the unheated lower level and the access corridor. In both scenarios, it was decided to activate the heating system for the entire building (8000 m²). The results show that the energy loss through open windows was significantly reduced thanks to the improvement of the transparent envelope components, adopted in the first scenario. This led to a decrease in energy demand per square meter heated, from 116.9 kWh/m² to 68.2 kWh/m². These results highlight the importance of sealing the openings in the unused areas of the villa, with a potential improvement in energy performance of about 40%. Additionally, the insulation of the external walls and floors helped to further reduce the energy demand for heating, bringing it down to 42.2 kWh/m², representing a 38% improvement in efficiency compared to the window-only intervention.

In some other cases, it was highlighted that, with the intervention on the building envelope, both opaque and transparent, the constrains due to the preservation of the historical value limited the energy savings achievable, which is the case of the Museum in Bolzano [14].

Concerning the transparent building envelope, wooden frames were widely used. Indeed, sometimes, for the reason of protecting the historical value of the building, the replacement of wooden windows is prevented. In such cases, it is possible to intervene through restoration, with grinding, planing, sealing with new gaskets and improving the air tightness, as well as replacing single glazing with double glazing: an example is given in Figure 3.

An aspect of great importance to consider is the formation of mold, which can compromise the preservation of historic buildings, particularly those housing museums. In this regard, several energy solutions have been proposed for the main building of the Taehan Medical Center in Seoul [15], which currently houses the Museum of Medicine and serves as an office space. These solutions include the thermal insulation of the opaque envelope (the roof and internal side of the vertical walls), the adoption of triple low emissivity glazing for transparent surfaces, adding internal curtains, reducing air infiltration and replacing traditional lamps with LED lighting. The package integrating all these technologies proved to be very effective, enabling a reduction in heating energy demand by up to 72% and overall energy savings of up to 60% (however, the cooling demand has almost doubled). In particular, the thermal insulation of the wall and roof significantly impacted mold control, lowering the mold index to extremely low, almost non-existent levels (the evaluation was conducted with WUFI 2D software, https://wufi.de/en/software/wufi-2d/, accessed on 24 November 2024). This improvement ensured greater safety for the collections on display and better preservation of the structures. The adopted solutions have thus proven crucial in ensuring the continued use and preservation of the building, bringing benefits in terms of both energy efficiency and historical–artistic conservation.

As highlighted in the discussed paper, the interventions on building envelope components are quite common and recurring. More innovative technologies, largely explored in the panorama of building energy retrofit, are not addressed for the historic buildings, such as opaque ventilated façades, transparent double skin façades, green façades, and the integration of phase change materials. Clearly, this is due to the cited limitations and restrictions that such types of buildings incur.

However, an innovative and sustainable technology that preserves the historical and architectural value of the building was presented in the study by Zazzini et al. [16]. The proposal suggests the installation of a green wall at the rear of the G. Pascoli Library in Foggia, in climatic zone D in Italy, as the main façade is subject to architectural restrictions. A self-supporting steel structure positioned 50 mm away from the main wall has been chosen to ensure effective interstitial ventilation and prevent moisture issues. The vertical garden system does not obstruct the windows, ensuring adequate natural lighting, and uses an integrated drip irrigation system. The results demonstrate significant energy savings for both cooling and heating. During the summer months, from June to September, the energy demand for cooling is significantly reduced, with a maximum electrical energy reduction of 35.2% in August. Furthermore, from December to February, the gas energy demand for heating decreases, with a maximum reduction of 35.4% in February. The analysis highlighted significant improvements in the comfort parameters. The indoor thermal comfort, related to the indoor air temperature, the mean radiant temperature, the air relative humidity, the air velocity, the metabolic rate and the thermal resistance of clothing can be evaluated through two indexes: PMV (Predicted Mean Vote) and PPD (Predicted Percentage of Dissatisfied). The first one provides an average measure of the comfort with reference to an indoor environment, while the second one estimates how many people are not satisfied with the microclimatic conditions of the environment.

In this study [16], the PPD index (Predicted Percentage of Dissatisfied) remains below 20%, and during the hottest (June–August) and coldest (December–February) months, it exceeds just 10%. Considering the costs and benefits of the intervention, the payback period for the initial investment is estimated to be around 7–8 years. To effectively assess the performance of the proposed technology, an economic analysis is required.

3.2. Interventions in Building Systems

Despite the high number of studies focusing on energy-efficient renovations of building envelope systems, it is often challenging and limiting to intervene directly on the building envelope due to the historical significance of certain buildings. For this reason, and to effectively achieve or move closer to the standard of Nearly Zero Energy Buildings, once all the possible interventions on the envelope are proposed, the focus usually shifts to upgrading the systems to improve energy efficiency and achieve significant savings.

Among the selected papers, there are few papers only discussing the systems for microclimatic control, because, in many cases, these interventions are coupled with the ones on the building envelope. Moreover, according to the authors, this is positive feedback because the objective at the European level is the reduction in energy demand and the consequent CO_2-eq emissions, to enhance the building resilience, and thus, also minimize the energy need and thermal discomfort under a free running indoor microclimate. The adoption of more innovative systems can help to reach such an objective, but the main aim is to reduce the energy demand at a prior level, such as in the construction phase for new buildings or with the proper design of retrofit solutions on the building envelope, and then implement innovative systems to satisfy the thermal comfort of the occupants.

An example is the Paleontology Museum of Naples, in climatic zone C in Italy, investigated in [17], for which several systems for microclimatic control are tested. The analysis showed that replacing the current obsolete boiler with a condensing one makes it possible to determine a reduction of about 18% in the primary energy demand, limited to the heating season. However, replacing the boiler with an electric air-to-water heat pump would achieve savings of around 45%. In the current situation, the museum does not include any system for the summer season, but if a reversible heat pump was chosen, combined with packaged air conditioning units for the rooms that currently lack both heating and cooling systems, the primary energy demand would be 97 kWh/m², with only a 5% increase compared to the current configuration. In addition to ensuring greater energy efficiency for the building, the proposed retrofit could improve thermal comfort without compromising the integrity of the building’s historical and artistic heritage. The theme of better temperature and thermo-hygrometric conditions contribute to the preservation of the museum’s exhibits. Indeed, according to the type of artifacts, the Italian regulations, UNI 10829, define the acceptable temperature and relative humidity to guarantee the preservation of the materials. Therefore, it is important not only to reduce the energy demand of the building but also to respect the prescribed threshold. This was also emphasized in [18]. An improvement of the heating systems was proposed for a Church in Italy, highlighting that a well-designed and proper distribution of hot air is critical, and a correct trade-off must be achieved between the comfort of the occupants of the environment and the preservation of the paintings and artifacts. For specific intended uses (for example, the Churches), a local control of the microclimatic conditions, besides the control of the whole indoor volume, should be preferred. This is approach is human-centered, based on the use of personal comfort systems (PCS) whose aim consists of improving “individual” comfort (instead of that of the entire environment) in terms of temperature, humidity, and therefore, air quality.

A comparison among three active energy systems is also proposed in [19]. The VRF system was revealed to be more efficient for a museum in Tehran than the air-cooled chiller and DX split systems, with savings on the energy demand of around 50%.

The interventions on the systems not only reduce energy consumption but can also lead to significant reductions in energy running costs, as demonstrated by the study conducted by Pochwala et al. [20] on a historic building in Woskowice Małe, Poland. The study compared two modernization solutions: the first involved the installation of six LPG gas heat pumps combined with a peak heat source consisting of a cascade of five gas condensing boilers; the second proposed four electric air-source heat pumps, together with an electric induction boiler as the peak heat source. The second option proved more advantageous, despite having a slightly higher net present value, achieving a reduction in heating costs of more than 72% (0.062 €/kWh) compared to the current coal system, surpassing the first solution, which achieved a 59% reduction (0.062 €/kWh). Both solutions proved to be environmentally friendly, with CO₂ emission savings of approximately 121.1 tons. Furthermore, if scenario 2 is combined with the installation of a more efficient lighting system and hot water production using the heat pump, the overall energy savings could reach 73% annually. Additionally, the heat pumps proposed in both scenarios, being reversible, can ensure indoor comfort in both the winter and summer seasons.

This technology demonstrates the possibility of significantly reducing the environmental impact, operating costs and energy consumption while still respecting existing architectural constraints. Further savings could be achieved through the adoption of photovoltaic systems, which, however, are not feasible in this context due to architectural restrictions.

3.3. Combination of Passive Retrofit Strategies and System Efficiency Improvements

When compatible with the individual case study, an integrated design of the enclosure components and plant systems can certainly bring more benefits in terms of the reduction in energy needs, and the consequent environmental impact, improvement of indoor thermal comfort conditions and savings on the operational costs.

The study by Ascione et al. [21] highlights that additional wall insulation can be almost unnecessary in historic buildings with masonry walls, characterized by their good thermal inertia properties which are capable of providing effective thermal insulation. For this reason, in the case of Palazzo Penne in Naples [21], the refurbishment primarily focused on improving the horizontal opaque structures (mainly the roof), the transparent envelope and the HVAC systems. By installing low-emissivity glazing and insulating the roof, the overall cost was reduced from 280 €/m² to 255 €/m², resulting in a 26% reduction in energy consumption and a 16% decrease in CO₂ emissions. Concerning the HVAC systems, the adoption of fan units powered by a condensing boiler and a high-efficiency air-cooled chiller is preferrable compared to a radiant system powered by heat pumps because of the high global costs associated with the latter. The HVAC solution, coupled with the interventions on the building envelope, ensures energy and environmental savings of 38% and a global cost reduction of around 20%. This solution offers an ideal balance between energy efficiency, emission reduction and economic sustainability, keeping global costs under control while representing a viable scenario for energy retrofitting that respects the historic constraints of the building; however, it is still far from the zero-emission standard.

In the study by Milvanovic et al. [22], focused on the rehabilitation of a historic building in Zagreb damaged by the earthquake and currently unusable, the need to combine energy efficiency improvements with seismic reinforcements emerged. To preserve the original appearance of the street-facing façades, materials like cork and thermal plaster are selected, while the ETICS system is used for the courtyard-facing façades. As an alternative to these insulation solutions for the opaque envelope, the option of applying mineral panels to the interior walls of both types of façades was also considered. With reference to the energy performance of the building, the thermal insulation of the opaque surfaces determines a reduction in heating energy demand of around 43–53%, with the energy retrofit of the transparent components such reduction increases of 14%. Additionally, the integration of a mechanical ventilation with heat recovery (MVHR) system brings energy savings up to 82% compared to the initial condition and improves indoor air quality.

Based on a hygrothermal analysis and an evaluation of thermal bridges, it has been determined that the most effective solution to prevent condensation and mold formation is the installation of an ETICS system on courtyard or non-visible façades, while cork-based thermal plaster should be applied to the façades facing the street. With this solution, coupled with the intervention on the transparent components, savings on the total energy demand of about 55% is achieved compared to the building’s pre-earthquake condition. This reduction can reach 67% when a mechanical ventilation system and high airtightness of the envelope are also included. Further improvements can be achieved by upgrading the existing technical system to a more efficient one, with an additional reduction in primary energy and emissions of up to 75%. Additional reductions could be achieved with the implementation of photovoltaic systems, which are pending approval by cultural heritage authorities.

The study demonstrates that a rehabilitation strategy combining energy efficiency, seismic safety and cultural heritage conservation can enable a hundred-year-old historic building to meet the national energy requirements for new buildings, providing concrete benefits for both the occupants and the environment.

High savings for both heating and cooling energy demands are also achieved for the Krsan Castel in Croatia [23] that is currently in ruins. Indeed, by coupling interventions on the building envelope (thermal insulation of walls, the roof and floor, as well as the installation of new windows with external shading devices) with a mechanical ventilation system with heat recovery and a strategy to use natural night ventilation, it is possible to reduce the energy demands for heating and cooling by about 80% and 70%, respectively. With respect to the previous study [14], an economic analysis is also discussed in [4]. Although the initial investment for the proposed energy retrofit measures is substantial, the energy savings achieved over time make the selected package effective, with a payback period of approximately 16 years.

In some climatic contexts, such as mountain or oceanic climates, a cooling system may not be strictly necessary. However, it is possible to limit summer overheating by implementing appropriate measures that meet the required standards. A significant example is provided by the study conducted by Besen et al. [24], which analyzed three buildings used as offices, classrooms and laboratories in three different locations in New Zealand. The most effective measures to meet EnerPhit standards (established by Passive House Institute) include the insulation of the roof, floor and internal walls, as well as the use of low-emissivity double-glazed windows with wooden frames installed inside the original windows. These interventions reduce heat loss and enhance occupant comfort. Additionally, installing a heat recovery ventilation system contributes not only to energy savings but also to improved indoor air quality. The study’s results indicate that, thanks to these measures, it is possible to achieve a reduction in heating energy consumption of up to around 90% in all climatic contexts. Furthermore, the interventions have significantly reduced the frequency of overheating hours—defined as hours when indoor temperatures exceed 25 °C—keeping this frequency below 10% in all of the analyzed buildings. Particular attention has also been given to moisture management, as the insulation of the internal walls, which is necessary for this type of building due to the restrictions on external walls, can present significant challenges. To address this, hygrometric simulations were conducted to identify the materials with the best performance, and calcium silicate panels were chosen for their ability to regulate moisture. This case study not only offers a detailed plan for the deep energy retrofitting of buildings but also provides an opportunity to effectively evaluate an integrated approach to both seismic and energy interventions, demonstrating the effectiveness of a synergistic approach in these two areas.

Another significant example of balancing the preservation of historical heritage with the energy retrofitting of a building is represented by the Castello di San Martino in Parella, located in the province of Turin. The study conducted by Serraino et al. [25] details the interventions carried out after a long period of abandonment of the complex. In addition to an extensive renovation that allowed for the reuse of spaces for various purposes, such as restaurants, a café and a hotel, the castle underwent fresco restoration and significant improvements to its building envelope. These included the renovation and insulation of the severely damaged roof and the replacement of some windows with low-emissivity double-glazed models with wooden frames.

The technical systems benefited from a significant technological upgrade, including the installation of water-to-water heat pumps, which use the aquifer as a thermal source for heating and cooling, and a cogeneration system capable of producing both electricity and heat, used for domestic hot water production. Additionally, many ventilation systems were equipped with high-efficiency heat recovery devices, further contributing to the reduction in energy consumption. Finally, an integrated monitoring system was implemented to optimize the overall management of the building. A proper management of the systems, coupled with the improvement of the efficiency of the building envelope components, was also recognized as very effective in [26]. Thanks to the combination of all these interventions, it was possible to achieve high levels of comfort and indoor air quality, significantly improve energy efficiency, reduce emissions and maintain operating costs. All of these were accomplished while fully respecting the strict architectural protection regulations, which prohibited interventions on the exterior walls, to preserve the historical appearance, and on the interior walls, due to the presence of valuable frescoes. The combination of passive and active energy retrofit measures in some cases is necessary to achieve proper energy, and therefore, environmental and economic—on the running costs—savings; this is the case of a hotel building located in Izmir, Turkey [27], for which the thermal insulation resulted to be not effective, and only by implementing efficient lighting and HVAC systems was it possible to achieve savings of around 53%.

3.4. Integration of Renewable Energy Sources in Energy Retrofitting Plans

The integration of energy retrofitting measures has proven to be highly effective in achieving elevated energy standards and significantly reducing energy consumption. The adoption of renewable energy sources—such as solar, wind and geothermal sources—offers considerable energy, environmental and economic benefits, contributing to the transition towards a more sustainable future. These technologies can enhance energy efficiency, lower pollutant emissions, and reduce overall energy usage. However, the implementation of renewable energy systems in historic buildings poses specific challenges due to the need for structural modifications and the limited space available, compounded by the strict requirements for cultural heritage preservation. In recent years, research has focused on developing tailored strategies to integrate renewable energy solutions into historic structures, which are widespread across Europe. These buildings face a dual challenge: addressing their typically high energy demand while safeguarding their historical and cultural value.

Several studies can be found in the scientific literature concerning the implementation of renewable energy sources in historical buildings.

For example, Piselli et al. [28] proposed the improvement of the HVAC systems using geothermal energy in one of the historic buildings of the medieval complex of Sant’Apollinare in Perugia, in climatic zone E in Italy, where an energy retrofit was already carried out to optimize both the opaque and transparent envelopes. The gas boiler was replaced with a water-to-water geothermal heat pump with a thermal power of 15 kW and equipped with geothermal exchangers. The water storage tank was retained, connecting it to both the radiant system and the air handling unit. The results obtained show a reduction in the energy demand for heating of up to 73% compared to the previous boiler, while maintaining the same operating conditions and comfort for the occupants. Additionally, significant economic and environmental benefits were recorded, with a 69% reduction in CO₂ emissions and a substantial decrease in operational costs. The proposed intervention, integrated with the previously implemented measures, demonstrates that the adoption of renewable sources can have a significant impact on the environment while ensuring the preservation of the building’s historical integrity.

In the work by Franco et al. [29], a feasibility study is presented on the Albergo dei Poveri in Genoa, focusing specifically on the use of biogas as a renewable fuel, coupled with traditional retrofit technologies, such as thermal insulation for the opaque components and the energy improvement of the transparent ones. The plan also involves replacing traditional lighting with LED lights to optimize the lighting system and installing natural gas microturbines to develop a cogeneration or trigeneration system. The thermal insulation of the structures allows for a significant reduction in the thermal transmittances, which, with the installation of new performing windows, could lead to energy savings of 32%. For lighting, a decrease in electricity demand of up to 50% is expected with the introduction of LED systems. The expected benefits of implementing micro-cogeneration include the reduction of CO₂, NO_x and other greenhouse gas emissions, as well as a decrease in natural resource consumption with biogas or by increasing efficiency via CHP systems. The plan could also include the creation of “energy islands” and the development of an advanced smart grid. An additional possibility is the integration of photovoltaic panels in the glass-roofed hanging gardens that enclose the valley of S. Nicola, which need to be preserved and restored.

Regarding solar energy, the most commonly used technology consists of the implementation of photovoltaic systems. However, these present various limitations from both an architectural perspective and in terms of visual and aesthetic impact, especially when applied to historic buildings.

The study by Mauri [30] on the building housing the Province of Agrigento exemplifies this issue, as building code restrictions in Agrigento prevent the installation of photovoltaic systems on the skylight, obstructing the achievement of more ambitious nZEB goals. The restrictions adopted for the city of Agrigento align with the views of experts in the field of architectural and historical preservation, according to which the photovoltaic systems, like the shading systems, have a high impact on the value of the building [31]. The results indicate that adopting conventional technologies, such as insulating internal walls with rock wool panels, replacing windows with more efficient alternatives, upgrading heat pumps to geothermal pumps and switching to LED lighting, can reduce energy demand by up to 30% (from 23.6 kWh/m³ to 16.2 kWh/m³), though this still falls short of the nZEB classification threshold. Only by supplementing these measures with the installation of monocrystalline silicon photovoltaic panels on the roofs and a photovoltaic skylight would energy consumption fall below 5 kWh/m³, with CO₂ emissions amounting to 1 kg/m³, thus meeting the nZEB target. Given these limitations, the installation of photovoltaic panels remains the only viable option for reducing energy demand below the 10 kWh/m³ threshold, bringing the building closer to nZEB standards. From an economic perspective, not all interventions are cost-effective. For instance, envelope insulation and replacing heat pumps with geothermal options are not financially viable unless combined with other measures, such as photovoltaic panels and LED lighting. However, this approach would significantly extend the payback period, and with market interest rate fluctuations, the economic outcomes in such scenarios become uncertain.

Some limitations related to the installation of PV panels are also highlighted in [32], where the case study is a residential property located in the center of Catania, which is subject to strict conservation constraints. Due to these restrictions, thermal insulation intervention was limited to the secondary façade of the building, along with roof insulation and the replacement of the existing windows with selective glass. These interventions contributed to an energy saving of around 19% and 32%, respectively, in the winter and summer seasons. Regarding the integration of photovoltaic panels, the project included the use of BIPV (Building Integrated Photovoltaics) tiles made with an amorphous silicon film, with an efficiency of around 5%. These tiles were installed exclusively on the east and south-facing roof slopes. This configuration resulted in an overall energy saving of only 8.9%, which was insufficient to meet the energy demand of the heat pump, but it did halve the electricity consumption due to lighting. The modest photovoltaic energy production is mainly due to the limited use of available surfaces, as only certain portions of the roof were involved, and the low efficiency of the material used. Although amorphous silicon has a 1much lower efficiency compared to monocrystalline silicon, it was chosen to minimize the aesthetic and visual impact, as well as to ensure greater flexibility and ease of integration of crucial requirements to comply with the restrictions imposed on protected properties.

These studies once again highlight how achieving critical energy goals for the ecological transition often requires the adoption of technologies that may conflict with regulations related to the preservation of historic heritage. It is, therefore, essential to find a balance between environmental protection and safeguarding cultural heritage by developing solutions that respect both needs.

An example of the effective integration of photovoltaic systems can be found in the case study by Jiang et al. [33], which presents a rehabilitation plan for the Jelinek house in Trieste, aimed at preventing the progressive degradation and abandonment of the building. The proposed measures include the application of insulating plaster on the internal walls (main building), the replacement of windows with more efficient ones (annex building), the installation of a heat pump powered by renewable energy sources and the integration of BIPV photovoltaic modules on the roof of the main building. Additionally, conventional photovoltaic modules will be installed on the roof of the annex building at a low inclination to minimize visual impact.

The results show that, although these solutions do not fully meet the energy demand, they cover about 46% of it, underscoring the importance of adopting such technologies to significantly reduce grid energy demand and promote self-sufficiency, contributing to a more sustainable future vision. This is also the case of the University of Seoul [34], where an improvement in all the three energy efficiency levels of a building, such as the building envelope, the energy-active systems and the integration of renewable energy sources, led to savings of around 54% and 43%, respectively, for the heating and cooling energy demand. Shifting the attention to a whole university complex, consisting of 19 building in China, the installation of PV panels, coupled with passive and active energy retrofit interventions, allowed it to meet the overall energy demand of the complex [35].

In Figure 4, an excellent example of solar renewable sources can be seen in a historical context of high value (historical center of ancient Italian city). In particular, the solar and photovoltaic panels, positioned at low tilt and set back from the perimeter of the buildings, serving impressive solar heating and cooling systems, are very well inserted into the context of the ancient square (roman ruins are present in the basement of some buildings), resulting in them being invisible to the view of the city.

A further example of an effective intervention based on the use of renewable energy sources is provided by the study conducted by Charalambous et al. [36]. A hybrid energy storage system is proposed for a building located in Aglantzia (Cyprus), integrated with a reversible direct current (DC) heat pump and used to ensure thermal comfort. At the same time, a hybrid AC-DC distribution system allows for the connection of the photovoltaic system, battery and electrical loads of the building. The photovoltaic panels, battery and DC-powered devices are connected to the DC section, reducing the losses caused by unnecessary conversions, while the alternating current (AC) loads are managed by the AC section. To maximize the storage and utilization of electrical and thermal energy from renewable sources, an electric storage system and a thermal storage system based on phase change materials (PCM) were selected, tailored to meet the specific needs of residential buildings, utilizing the most efficient technologies currently available. The results indicate that the proposed system achieved significant energy savings, with approximately a two-thirds reduction in imported energy from the power grid compared to a conventional building. Another noteworthy aspect is that over 85% of the energy consumed by the building came from renewable sources, a result achieved through the combination of the photovoltaic system and energy storage. This approach not only increased self-consumption but also greatly improved thermal comfort in both winter and summer: during the summer months, the building remained entirely within the thermal comfort limits, without any overheating events, even during the hottest periods, while in the winter, the system maintained optimal indoor temperatures with a reduced need for additional heating hours. Therefore, the approach contributed to the improvement of the indoor thermal comfort and guaranteed energy efficiency, reducing the operational costs and environmental impact. Furthermore, the benefits extend to both the environmental and economic spheres, with a reduction in energy costs and greenhouse gas emissions, thanks to the decrease in the purchase of electricity from the grid. The improvement of indoor thermal comfort and a reduction in the energy demand of the building is also achieved in [37] thanks to the enhancement of the building envelope performance, coupled with the substitution of the systems for microclimatic control and the integration of renewable energy sources.

Ozbalta et al. [38] propose a series of interventions for the historic building Palazzo Ahmet Aga, a public office built in 1800 in Izmir, Turkey, which is subject to cultural heritage conservation restrictions. Energy simulations show that internal wall insulation with rock wool can reduce total energy consumption by 2.6% to 3.3%, with greater savings at higher thicknesses, but with a corresponding increase in costs. Secondary windows proved more effective than low solar gain glass, reducing energy consumption by up to 5.16%. The introduction of an LED lighting system reduced the energy consumption for lighting by 21.3%, contributing to overall savings of around 2%, making it a cost-effective choice. From the system perspective, the three-pipe VRV system is the most efficient, reducing total energy consumption by 11.5%, which is particularly relevant for cooling, while the air-source heat pump is less efficient, reducing consumption by 4.6%. The introduction of photovoltaic panels represents the most effective solution. The converted energy covered about 14.4% of the electricity demand of the building by lowering the purchase of energy from the grid, and therefore, the costs, by about 10.7%.

The optimal combination includes the use of 5 cm insulation, secondary windows, LED lighting, the adoption of the three-pipe VRV system and the installation of photovoltaic panels. This configuration leads to a total energy consumption reduction of 34%, with a consumption of 167.24 kWh/m², and a reduction in overall costs of 15.7%, corresponding to a global cost of 775.5 €/m².

The study also highlighted that the standalone application of some technologies, such as attic and basement floor insulation, results in minimal energy savings relative to the costs incurred. This emphasizes the importance of addressing multiple aspects of buildings to achieve significant benefits in both energy and economic terms. The results also demonstrate the crucial role of renewable energy sources, which represent the solution with the greatest energy and economic impacts among those analyzed.

When evaluating the feasibility of an energy retrofit on a building, it is crucial to consider not only the energy performance and economic parameters but also the environmental impact through a Life Cycle Assessment (LCA). A study by Prieto et al. [39] on a historic office building in Gijón, Spain, which is under integral protection, analyzed different types of interventions, ranging from the building envelope to renewable energy sources and technological systems. The research highlighted that it is not necessary to intervene on multiple fronts to achieve benefits, as not every energy-efficient or economically advantageous measure is sustainable over time. From an energy perspective, interventions on the envelope (such as internal wall insulation with mineral wool and the replacement of wooden frames and double-glazed windows with aluminum frames featuring thermal break and triple-glazed argon-filled panes) significantly reduced energy consumption, from 59.2 kWh/m²⋅year to 43.1 kWh/m²⋅year. The installation of photovoltaic panels and heat pumps increased the use of renewable energy, but it would not have been necessary if the envelope improvement had been carried out properly.

From an economic standpoint, the installation of photovoltaic panels alone showed the most favorable payback period at 11.2 years. The scenario with the best net present value (NPV) and return on investment (ROI) combined photovoltaic panels and heat pumps. Although envelope improvements have longer payback periods, they represent a more sustainable investment in the long term.

From an environmental impact point of view, the improvement of the envelope and the adoption of an energy mix based on renewable sources proved to be the optimal solution. Although active systems are more economically advantageous, interventions on the envelope significantly reduce the environmental impact, with the energy savings compensating for, up to seven times, the impacts caused by the materials used. Therefore, this solution is the most sustainable in the long term, both in terms of energy efficiency and emission reduction.

The LCA approach implemented in [40] for a residential and office building in Coimbra, Portugal, revealed that the thermal insulation intervention involved the highest impact compared to both window and HVAC components, especially in crowded residential environments; therefore, a correct design of the energy retrofit interventions is needed in order to avoid the excessive weight of the embodied energy and carbon emissions.

Also, in the case study by Tiberi et al. [41] on the pharmaceutical chemistry building at La Sapienza University in Rome, built in 1962, the environmental impact and energy performance are key factors in determining the best approach for refurbishment interventions. In the first proposed scenario, improving the building envelope (replacing windows, insulating the roof and ground floor and insulating the external walls) reduced heat losses by 54%, resulting in an energy savings of 256.7 MWh/year. The most energy-efficient scenario is the one that combines envelope improvement with a 20 kW photovoltaic system and thermostatic valves, reducing heating demand by 91% and electricity consumption by 1%, for a total energy demand reduction of 42%. However, this intervention comes with a long payback period, exceeding 30 years, and requires an investment of over one million euros. The most economically viable scenario involves installing a 50 kW combined heat and power (CHP) system and a 20 kW photovoltaic system, without any improvements to the building envelope, with a payback period of 15.9 years. Despite offering lower energy savings, it is the only scenario with an acceptable economic return. However, the cogeneration system proves less advantageous for buildings of this type due to the high maintenance costs and limited performance.

The thermal insulation of walls is often essential for improving the energy efficiency of many buildings, but in some cases, it may be unnecessary, especially when the existing structures already provide a low level of thermal transmittance. This occurs when the original walls are sufficiently thick or made of materials with high thermal inertia, which are capable of maintaining good indoor comfort without the need for additional interventions on the building envelope. Such interventions could also conflict with architectural or historical constraints.

A significant example is the Albergo dei Poveri building in Naples, constructed in the second half of the 18th century and studied by Bellia et al. [42], where the thermal properties of the original tuff walls rendered additional insulation unnecessary. As a result, alternative interventions for the building envelope were proposed: replacing the windows with high-performance ones, thermally efficient models with low visual impact and refurbishing the seventh-floor slab. These measures resulted in a 27.1% energy saving over the entire heating season. Regarding the use of renewable energy, the installation of photovoltaic panels proved particularly effective. Among the options considered (semi-transparent and opaque coverings), the opaque photovoltaic roof emerged as the preferred choice, increasing energy conversion by 20% compared to the semi-transparent option, providing 107.9 MWh/year of electricity. This type of roof not only converts more energy but also mitigates the risks of glare and summer overheating, issues that could arise with a semi-transparent roof. Although the latter slightly enhances natural light, it does not significantly improve visual comfort, making the opaque roof the more advantageous solution overall. To ensure thermal comfort, along with reducing heat loss through envelope improvements, a radiant panel heating system was proposed, as the building currently lacks a heating system. For the summer season, indoor thermal comfort is guaranteed by improving on a well-designed natural ventilation [14] that naturally regulates the indoor air temperature and humidity, preventing indoor overheating, and leveraging the thermal inertia of the walls.

As in previous studies, the environmental impact is crucial to the feasibility of these interventions, which demonstrate a positive outcome with a reduction of 32.1 tons of CO₂ emissions.

A significant example of a successful deep energy retrofit in a historical context is represented by the Ca’ S. Orsola building [43], located in the heart of Treviso’s historic center in northeastern Italy. Originally a convent, the building was purchased in 2007 and underwent a comprehensive renovation, transforming it into a prestigious residential complex. Despite the restrictions imposed by the Veneto Regional Authority for Historic and Architectural Heritage, targeted and respectful interventions achieved remarkable results in seismic safety, energy efficiency, environmental impact, economics and living comfort.

The renovation addressed both the opaque and transparent elements of the building envelope, utilizing expanded polystyrene foam, rigid mineral wool panels covered with plasterboard, a new wooden roof structure insulated with wood fiber and waterproofed, and double-glazed, low-emissivity windows with wooden frames. In terms of systems, two heat pumps were installed: the first uses an underground well as a source of hot/cold water and acts as a chiller, while indoor comfort is ensured by a radiant floor system and summer dehumidification; the second heat pump is dedicated to domestic hot water (DHW) production. Mechanical ventilation is provided by a heat recovery system. For renewable energy, solar thermal panels were installed for DHW production, complemented by photovoltaic systems on the roof.

Thanks to these interventions, the overall energy consumption was reduced by 92.5%, with an 88% reduction in heating demand, a 24% reduction in DHW demand and a 56% drop in electricity consumption. In addition to energy savings, the renovation achieved excellent environmental results: greenhouse gas emissions were reduced by 81%, from an equivalent of 29.8 kg of CO₂ per m² per year to 5.8 kg of CO₂/m². Although the renovation costs were substantial, the investment value was validated by the increased market value of the property and long-term energy savings, with a payback period estimated to be 13 years.

Special attention was also given to thermal and acoustic comfort, resulting in a high-quality living environment with controlled temperature, excellent air quality and superior sound insulation (class 1 acoustic rating). The results transformed the building into a nZEB (Nearly Zero Energy Building) with an A+ energy rating, demonstrating that it is possible to honor historical and architectural heritage while significantly improving the building’s energy and environmental performance, particularly through the extensive use of renewable energy sources.

4. Urban Scale: Energy Retrofit of Historic Buildings

To better assess the impact of energy retrofitting on historic districts or neighborhoods, recent studies are increasingly focusing on these specific case studies. The interventions proposed in the research over the past years involve both individual buildings and entire neighborhoods or cities, allowing for an analysis of the impact that improvement measures can have on the entire community.

In neighborhoods characterized by buildings from different styles and construction periods, two distinct intervention packages could be considered: a less invasive one for traditional buildings and a more invasive one for more modern structures, as demonstrated by Sugar et al. [44] in their study on a district in Budapest. The less invasive intervention includes the insulation of the roof, internal walls without decorations, the cellar slab and upgrading windows with low-emissivity glazing. For buildings constructed after World War II, which are subject to fewer architectural restrictions, the intervention also involves insulating external walls where possible, insulating the ground floor slab and completely replacing windows with more efficient models. These interventions, combined with the adoption of a heat pump for heating, cooling and domestic hot water production, result in a 69% energy saving for heating and DHW production. Furthermore, these solutions enable 85% of buildings to meet Nearly Zero Energy Building (nZEB) requirements, compared to the initial 4.7%. The analysis demonstrates that it is possible to significantly improve the energy performance of historic buildings without compromising their architectural value. The proposed interventions successfully balance energy efficiency, environmental sustainability and cultural heritage preservation, offering replicable solutions in other similar urban contexts.

A similar approach was adopted by Belpoliti et al. [45] to reduce the energy consumption of Caporciano (L’Aquila), a town partially uninhabited due to earthquake damage. For the oldest historical buildings, a conservation intervention was planned, including the internal insulation of the walls and ground floor, upgrading windows with low-emissivity glass, installing a condensing boiler and adding a thermostat. For buildings where a more architecturally impactful intervention was feasible, roof and external wall insulation were also added. Thanks to this strategy, a substantial reduction in energy consumption was achieved, lowering it from the current 4.91 GWh to 2.33 GWh, based on the current occupancy rate of 36%. According to municipal forecasts, renovating the buildings could stimulate social and economic revitalization in Caporciano, increasing the occupancy rate by up to 60%. Under this scenario, consumption would rise to 3.77 GWh per year, which remains below the current 4.91 GWh. Even if the town were to be fully repopulated, energy consumption would reach 6.30 GWh per year—a still favorable outcome compared to the estimated 10.78 GWh for full occupancy without interventions.

In the study conducted by Teso et al. [46] for the Santa Marta district in Venice, four energy retrofit measures were analyzed individually and in a combined scenario to meet the thermal transmittance values required by the Italian legislation (Ministerial Decrees of 26 June 2015) for the access to state incentives. The measures examined include wall insulation, roof insulation, the replacement of the existing boiler with a condensing boiler and the replacement of single-glazed windows with double-glazed units. The findings indicate that adopting individual measures, excluding wall insulation, results in energy savings of 5% for both window replacement and roof insulation, while boiler replacement offers savings of 12%. However, the most effective intervention is wall insulation, achieving 55% in energy savings using EPS panels applied either internally or externally, depending on regulatory constraints. The combined implementation of all retrofit measures results in an overall energy consumption reduction of 67%, corresponding to a decrease of 1.1 MtCO₂ emissions avoided per year, which significantly contributes to air pollution reduction. From an economic perspective, the benefit is represented by energy cost savings reaching 67% if all interventions are implemented.

In historic districts where architectural restrictions are not particularly stringent, it is possible to promote the integration of renewable energy sources in synergy with CO₂ emission reduction strategies.

From the previous sections, the difficulties to reach high energy standards because of the restrictions in the matter of historical buildings were underlined. However, when working with urban districts, it is possible to install PV technology in the surroundings of the district, without altering the historical value of the buildings. This was the example of an ancient village in Saudi Arabia, which was able to reach the standard of nZEB [47].

A further notable example is the neighborhood in Rome studied by Tajalli-Ardekani et al. [48]. In this case, alongside traditional thermal insulation measures and replacing lamps with LEDs, the installation of photovoltaic panels, batteries, high-efficiency cogenerators and absorption chillers, all managed by an intelligent control system, achieved remarkable results: a 55% reduction in primary energy consumption and a 56% reduction in CO₂ emissions, and a 68.2% reduction in energy purchased from the grid. However, the payback period was estimated at 20 years, which was longer than optimal, due to the high costs associated with the building renovations.

An alternative solution, which also considers the need to limit the summer overheating that may result from insulation interventions, was adopted by Blazquez et al. for a historic residential complex in Seville, Spain, located in the Mediterranean region [49]. The study focuses primarily on indoor comfort, both in summer and winter. Therefore, in addition to upgrading window components and thermally insulating interior walls, it is essential to integrate a controlled ventilation system with heat recovery to address increased discomfort during the summer season. The study found that the reduction in overall energy demand across all orientations (up to a 47% decrease) is more substantial than the improvement in comfort conditions, which ranges from 20% to 40% in winter and 35% to 50% in summer, according to the AACE standard. Under the EACE standard, comfort improvement ranges from 12% to 21% in winter and from 29% to 50% in summer.

5. Discussion

In this section, the revised articles will be discussed, aiming to highlight the limitations, benefits and prospects in the matter of the energy retrofitting of historic buildings.

In Table 1, the reviewed studies are summarized, providing a brief description concerning the investigated technology, the type of analysis conducted and the key points of each study.

The selected articles represent a broad range of possible interventions and provide analyses not only from an energy perspective but also regarding environmental impact, economic sustainability and comfort. The potential interventions range from more traditional solutions, such as the insulation of vertical walls, roofs and floors, improving glazed surfaces and optimizing active energy systems, to more innovative technologies related to renewable energy sources, such as photovoltaic and geothermal energy. Each intervention is examined from various perspectives, with the objective of assessing its applicability and potential for replication in similar contexts, while ensuring respect for the historical and architectural features of the buildings.

Among the 46 investigated studies, 33 are located in Europe and 22 in Italy. This confirms what is acknowledged in [3,4], namely that historic buildings are a defining feature of Europe and that their preservation, while simultaneously renovating and adapting them for new uses, is advisable.

As emerged from the review study, the thermal insulation of the building envelope was primarily employed to reduce the heating energy demand and improve indoor comfort during the winter season. However, this type of intervention may introduce the risk of summer overheating, causing discomfort for occupants and/or workers, as observed in studies conducted by Aparicio-Fernandez et al. [6], Qu et al. [9], Blecich et al. [23] and Cho et al. [15]. These studies demonstrate that envelope insulation reduces heating energy demand by over 50%. The improvement of the thermal performance of the building envelope must be linked to the proper management of the building [26], avoiding the cancelation of energy efficiency through the incorrect behavior of occupants or improper use of the building itself. Generally, different solutions have to be adopted according to the climatic conditions, as also highlighted in [12].

The thermal insulation of the building envelope in historical buildings has to be conducted with high care, attention and a careful design because the original structures are usually characterized by specific parameters. Indeed, the materials employed in these types of constructions are typically able to correctly manage moisture and mold formation. The addition of an insulating material can alter and adversely affect this behavior, and therefore, a correct selection is recommended [11,24].

Furthermore, during summer periods, excessive thermal insulation can be responsible for an increase in cooling energy demand compared to the baseline case. Nevertheless, overall, significant annual energy savings are achieved, especially in climates where winter months require greater attention.

Indeed, in certain geographic areas and specific contexts characterized by a cold and harsh climate (North Europe, Russia and some parts of Canada), it may be advisable to prioritize heating demands, relying on natural ventilation or suitable ventilation systems to limit summer overheating. In locations such as Nottingham [9], where summers are not particularly hot, or buildings that are primarily used during winter, such as in [6], where the building investigated is a school, and therefore, tends to be vacant in July and August.

In some other cases, the masonry structure plays a fundamental role in ensuring high thermal inertia, helping to maintain summer comfort even when thermal insulation interventions are implemented—or even rendering these interventions ineffective. For example, the study by Ascione et al. [21] reveals, through a cost-optimal analysis, energy, economic and environmental benefits by focusing efforts on improving the transparent envelope and system efficiency rather than wall insulation. Similarly, the study by Bellia et al. [42] highlights how the thermal inertia of walls can eliminate the need for wall insulation investments, instead prioritizing window improvements and photovoltaic systems to support the decarbonization process. In Besen et al. [24], interventions on the opaque envelope, again, significantly reduced the energy consumption for heating, while summer overheating was managed through targeted measures on window openings and the introduction of a heat recovery ventilation system, which also improved indoor air quality. However, unlike other previously analyzed cases, no economic analysis was conducted to assess the feasibility. Nonetheless, this intervention remains a valuable example of an integrated approach to energy retrofitting combined with seismic adaptation, as also demonstrated by Milovanovic et al. [22].

In [8], the results confirm an enhancement in thermal comfort in the winter season due to wall and roof insulation. However, in this case, summer discomfort was significantly reduced by upgrading to more efficient windows and implementing shading systems. Conversely, Blecich et al. [23] tackled overheating by drastically reducing the energy demand for both heating and cooling through interventions on the transparent envelope and the addition of a mechanical ventilation with heat recovery system. The intervention of mechanical ventilation has a crucial role even with respect to the topic of correct air distribution, and therefore, helps to guarantee proper breathability in the building.

Unlike previous studies [6,15], which did not perform an economic analysis, a payback period of over 30 years was calculated in the first case [8], highlighting the need to integrate renewable energy sources or increase financial incentives for a more sustainable intervention. Similarly, an economic analysis is proposed in [23], where a payback time of approximately 16 years was achieved, emphasizing the effectiveness of combining envelope improvements with retrofitting active systems. Ascione et al. [7] also demonstrated an acceptable payback period of around 15 years, due to the durability and effectiveness of integrated measures, while Lucchi et al. [11] achieved a PBT of less than 5 years by focusing solely on internal wall insulation, approaching the Nearly Zero Energy Building (nZEB) target. Economic analysis is strictly necessary to assess the effective convenience of an intervention.

The need for properly designed energy retrofit interventions has emerged, with a tailored approach that considers both the specific case study and the climatic conditions, which highly affect the building’s energy needs and the weight of heating and cooling energy demand in the annual balance.

Taking into account the objectives set at the European level regarding energy efficiency in buildings, and as discussed through the review of the selected studies in the scientific literature, the integration of renewable energy sources is essential to achieve goals such as nZEBs. However, the introduction of such systems in historic buildings is particularly challenging due to the constraints of heritage preservation. In the various articles analyzed, there were cases where these systems were successfully integrated, achieving a good compromise between respecting architectural restrictions and obtaining energy benefits.

A significant example is the intervention proposed by Piselli et al. [28], involving the installation of a geothermal heat pump capable of covering more than 73% of the heating energy demand. The study also includes a detailed analysis of costs and emissions, showing positive results on both fronts, which underscores the overall favorable impact of adopting renewable energy sources from both economic and environmental perspectives. The studies by Franco et al. [29], Lin et al. [35], Charalambous et al. [36], Ozbalta et al. [38], Tiberi et al. [41] and Dehwah et al. [47] clearly demonstrate the energy and environmental benefits of using renewable energy-based systems. In particular, Ozbalta et al. [38] emphasize that this solution can be more advantageous than traditional interventions in terms of both energy efficiency and cost-effectiveness. However, in the case of the University of ’La Sapienza’ in Rome [41], the most efficient scenario in terms of energy and environment is hindered by a prohibitively high initial investment, with payback times exceeding 30 years. A possible solution to reduce these costs could be the greater integration of renewable energy sources supported by targeted incentives.

Charalambous et al. [36] also highlight the importance of adopting storage systems, which are essential for ensuring service continuity and enhancing energy self-sufficiency. On the other hand, Prieto et al. [39] demonstrated that for a building located in Gijon (a temperate oceanic climate zone), after improving the building envelope performance, the installation of active systems such as photovoltaic panels and heat pumps did not result in a significant improvement in sustainability, making them redundant. As a result, a general and common approach has not been identified.

Given these findings, it appears that the adoption of renewable energy sources does not always provide an effective balance between performance and cost. Therefore, their installation should be based on an in-depth analysis that considers multiple factors.

The studies by Moschella et al. [32], Mauri [30] and Jiang et al. [33] focus on the adoption of photovoltaic panels, analyzing both the advantages and limitations of this technology. A critical issue that emerged is the challenge of preserving historical heritage, where the installation of panels may be restricted due to placement limitations and compatibility concerns with existing materials. The positioning and quality of the panels significantly impact energy production, raising questions about the effectiveness of this technology in historically significant buildings.

The limitations posed by local and national authorities with regard to the adoption of photovoltaic systems in historical buildings, are in line with the ideas of the experts in the field, according to which this is the most impactful intervention on historical value [30,31]. However, it was demonstrated that is the only solution able to guarantee to almost nullify the energy demand [35,47]. The ideal optimal solution, when feasible, is off-site installation [47].

Consequently, the future adoption of photovoltaic panels in these contexts will largely depend on technological advancements and regulatory agreements that reconcile cultural heritage preservation with the need for a sustainable energy future.

The investigation also focused on studies that involved interventions solely on the systems of historic buildings. These buildings, due to their structural characteristics, do not allow for extensive modifications, which is why solutions that neither alter the structure nor the external appearance are preferred. Notable examples include the Museum of Paleontology in Naples [17] and Woskowice Małe [20]. Otherwise, the need to find a right balance between indoor thermal comfort and the preservation of paintings or artifacts prioritized the installation of systems [18,19] to guarantee a correct distribution of the air inside the environments and an appropriate control of indoor humidity. A similar result was achieved also in [50] across several and future weather conditions.

In the case of the museum [17], the installation of a reversible heat pump enabled greater energy efficiency and improved thermal comfort in both the summer and winter seasons. However, it is important to note that, in this case, no cost analysis or comprehensive environmental analysis was conducted, focusing exclusively on the benefits in terms of energy efficiency and the preservation of historical and artistic heritage.

For the building in Woskowice Małe, the cost and environmental benefit analysis shows that transitioning from a coal-based system to a more sustainable solution not only brought significant energy benefits but also reduced heating costs, by over 72%, and CO₂ emissions.

In both cases, the proposed system solutions have proven capable of meeting the energy efficiency and decarbonization requirements, which is in line with European climate policies, contributing to the sustainability of historic buildings.

Regarding the articles addressing the energy retrofitting of neighborhoods and entire cities, all the selected studies highlight interventions that reduce energy consumption by over 50%. One of the most significant case studies is that by Tajalli-Ardekani et al. [48], which demonstrated substantial benefits from the implementation of innovative solutions based on renewable energy sources. However, the application of these measures presents important limitations: for instance, photovoltaic panels can only be partially installed to preserve the aesthetics and visual impact of the buildings, thus preventing complete energy coverage from renewable sources. Additionally, the proposed trigeneration system was designed for individual building blocks, as the historic nature of the neighborhood does not permit the installation of new underground piping required for the heat exchange between buildings.

In the other studies examined [44,45], innovative or renewable technologies could not be adopted due to the restrictions imposed by architectural heritage conservation, limited space and inadequate infrastructure. Despite this, the proposed interventions still improved energy efficiency and residential comfort [49], although they lack a comprehensive economic analysis, which is essential for assessing the true feasibility of these interventions. Given the large-scale nature of such projects—whether for neighborhoods or entire cities—the economic and environmental aspects become crucial. In this context, the study by Teso et al. [46] justifies a substantial initial investment due to the significant benefits in terms of energy efficiency, environmental impact and reduced operational costs. One limitation identified in the study concerns the thermal insulation of external walls in the historical center of Venice, which is protected by the Superintendence of Archeology, Fine Arts, and Landscape. In this case, applying external insulation is hardly feasible, making it necessary to install insulating panels within the buildings—a solution that reduces living spaces and may inconvenience occupants. Despite these challenges, the interventions aim to develop a refurbishment plan that is replicable and adaptable to other contexts.

Overall, it was emphasized that there is a critical theme in terms of limitations when working with energy efficiency in the sector of historical buildings. Indeed, there are several limitations due to the combination of legislative, cultural and technical aspects. As emphasized in most of the analyzed papers, the main restriction in the design of an energy retrofit intervention is related to the national or local regulations aimed to preserve the building in their original aspect and, above all, to avoid the damage of the historical components because there could be structural problems. Indeed, a proper analysis on the structural resistance of the building with the added materials should be conducted. Furthermore, the compatibility of the modern materials and technologies should be carefully assessed, especially to guarantee the correct maintenance of the artifacts and paintings commonly presented in such types of buildings [51]. The research of a right trade-off to balance all these aspects should usually be approved by the local or national authorities. For all these reasons, such interventions often involve long lead times, without taking into account the high investment costs and additional maintenance costs.

Table 1. Reviewed studies: investigated technology, type of investigation, location, type of building and main findings.

Year	Authors	Technology	Type of Investigation	City	Type of Building—Year of Construction	Main Findings and Results
2023	Aparicio-Fernandez et al. [6]	Opaque envelope: Thermal insulation of all components Transparent envelope: Replacement of window components	Experimental and numerical	Valencia (Spain)	Music school—1921	Energy demand can be reduced by 55.2%, reaching the Passivhaus standard. Heating demand decreases; however, it will be necessary to implement increased natural ventilation in the summer season to contain the rise in energy demand for cooling.
2015	Ascione et al. [7]	Opaque envelope: Internal wall insulation and insulation of roof Transparent envelope: Replacement of window components HVAC: Zonal thermostats	Experimental and numerical	Benevento (Italy)	Administrative and didactic buildings—1927	The proposed scenario leads to a 38% reduction in the annual primary energy demand and in greenhouse gas emissions. The payback period is around 15 years. Furthermore, the optimal management of microclimate control systems resulted in electricity savings of 24%.
2019	Galatioto et al. [8]	Opaque envelope: Internal insulation of walls and roof Transparent envelope: Replacement of window glasses and shading devices Lighting: LED lamps and lighting control	Numerical	Cagliari, Roma, Milano, Palermo (Italy)	Office—1938	The most effective intervention consists of internal thermal insulation, roof insulation and the replacement of windows with shading devices, involving energy savings of about 51% in Palermo, 52% in Cagliari, 56% in Rome and 43% in Milan. The chosen scenario allows the highest reductions in CO2. However, the payback time is over 30 years.
2021	Qu et al. [9]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of existing components	Numerical	Nottingham (UK)	Victorian-style house—1884	The optimal cost–benefit solution leads to a 51.8% reduction in primary energy demand with a payback period of around 18 years. While winter comfort improves, summer temperatures rise by 2.6 °C, so adequate ventilation is recommended.
2024	Yuk et al. [10]	Transparent envelope: Application of secondary windows	Numerical	Seoul (Korea)	University office—1924	The adopted solution involves a saving of around 17% and 24% in cooling and heating energy demand, and determines more stable conditions in terms of indoor air temperature and relative humidity.
2017	Lucchi et al. [11]	Opaque envelope: Internal wall insulation	Numerical	Como (Italy)	Residential villa—XVIII Century	All the proposed materials are energy-effective as internal insulation materials, while economic convenience is not always achieved, especially with the greater thicknesses of cork, perlite and foam glass. Mineral glass wool emerged as the most cost-effective material. With the intervention, the building is close to Nearly Zero Energy Building (nZEB) standards.
2015	Tadeu et al. [12]	Opaque envelope: External wall insulation, roof and floor insulation Transparent envelope: Replacement of window components HVAC: Substitution of heating and domestic hot water systems	Numerical	Coimbra (Portugal) and other Portugal localities	Commercial and residential—not declared	The main results concerned the thickness of the insulating materials. Therefore, the optimal thickness changed according to the Portugal localities, which differ for the heating degree days value. A higher thickness is required in cities with higher HDD. Moreover, an LCA analysis is conducted, and it revealed a balance between the embodied and operational impacts.
2016	Cornaro et al. [13]	Opaque envelope: Insulation of external walls and basement Transparent envelope: Replacement of window components	Experimental and numerical	Rome (Italy)	Villa Mondragone (conference center and foresteria)—1573	The replacement of the windows reduced the energy demand from 116.9 kWh/m2 to 68.2 kWh/m2. Subsequently, the insulation of the external walls and floors further decreased the heating energy consumption to 42.2 kWh/m2.
2017	Roberti et al. [14]	Opaque envelope: Internal and external walls and roof insulation Transparent envelope: Replacement of window components Natural ventilation	Numerical	Bolzano (North Italy)	Medieval building transformed in museum—end of the twelfth century	The optimal energy retrofit solutions are identified through a balance between the energy efficiency and the preservation of the building. The last is identified through a survey of the experts of heritage conservation. The results highlight that the refurbishment involves high energy savings, but further improvements will lead to a degradation of the building.
2020	Cho et al. [15]	Opaque envelope: Insulation of internal walls and roof Transparent envelope: Replacement of window glasses and internal blinds Lighting: LED lamps	Experimental and numerical	Seoul (Korea)	Museum and office (Taehan Medical Center)—1907	The package integrating all proposed technologies has proven highly effective, reducing the heating energy demand by up to 72%. The insulation of the walls and roof has drastically minimized the risk of mold, bringing the mold index down to very low, thereby enhancing the safety and preservation of the displayed collections.
2018	Zazzini et al. [16]	Opaque envelope: Installation of a green wall at the back of the library	Numerical	Foggia (Italy)	Library—1858	The intervention involves a reduction of approximately 17.8% in electricity demand and 28% in gas usage, with a positive impact on the residents’ comfort as well. The payback period has been estimated to be around 7–8 years.
2017	D’Agostino et al. [17]	HVAC: Condensing boiler, heat pump, chiller, reversible heat pump, packaged air conditioning units	Numerical	Naples (Italy)	Paleontology museum—between 5th and 7th century	The most energy-efficient solution is the use of a reversible heat pump that allows for the guarantee of an acceptable thermal comfort in the summer period as well, resulting in a primary energy demand of approximately 97 kWh/m2, with only a minimal increase of 5% compared to the current consumption (which does not include cooling).
2010	Camuffo et al. [18]	HVAC: Substitution of the heating system	Monitoring	Rocca Pietore and Santo Stefano di Cadore (Italy)	Church—15th century	A proper distribution of the heated air is required in order to guarantee acceptable conditions of air temperature and relative humidity for the occupants, avoiding a deterioration of the artifacts and paintings, and therefore, the accumulation of hot air in these areas.
2024	Etemad et al. [19]	HVAC: Air cooled chiller, variable refrigerant flow, split DX systems	Numerical	Tehran (Iran)	Museum—1933	The VRF system is the most effective with the lowest annual cooling electricity demand, with a reduction of around 50% with respect to the current situation.
2023	Pochwała et al. [20]	HVAC: Six LPG-powered air-source heat pumps and five gas condensing boilers, four electric-powered air compressor heat pumps and one electric induction boiler	Numerical	Woskowice Małe (Poland)	Rehab treatment facility—1800, reconstructed in 1912	The adoption of four electric heat pumps is preferable to that of six LPG heat pumps. This advantage is due to a greater reduction in heating energy costs (72% compared to 59%) and overall energy savings exceeding 70%. Both scenarios are environmentally friendly, ensuring a reduction in CO₂ emissions of around 121.1 tons.
2015	Ascione et al. [21]	Opaque envelope: Internal/external wall insulation and insulation of roof Transparent envelope: Replacement of window components HVAC: Several types of boilers, chiller (air-cooled), several types of heat pumps	Experimental and numerical	Naples (Italy)	Palazzo Penne (office)—1406	The optimal package consists of installing fan-coil units powered by a condensing boiler and a high-efficiency air chiller, combined with roof insulation and the adoption of selective or low-emissivity glass. This approach ensures energy and environmental savings of 38% and reduces overall costs by approximately 20%.
2022	Milovanović et al. [22]	Opaque envelope: External wall insulation Transparent envelope: New secondary windows, old existing windows (repaired and sealed) HVAC: Heat recovery ventilation system, condensing boiler	Numerical	Zagreb (Croatia)	Multi-residential building—1920	The results show that interventions on the building envelope can reduce primary energy consumption by up to 53%. Including mechanical ventilation and improving airtightness, the reduction can reach up to 67%. By also replacing the technical system with a more efficient one, the energy demand and emissions can be reduced by up to 75%.
2016	Blecich et al. [23]	Opaque envelope: Thermal insulation of opaque components Transparent envelope: Replacement of window components External shading HVAC: MVHR	Numerical	Istria (Croatia)	Castle of Krsan (currently abandoned, new use: tourist residences)—1274, subjected to several changes	Envelope insulation, efficient windows, improved airtightness and the MVHR system can reduce heating demand by over 80%. Strategies, such as external shading and night ventilation, can reduce cooling energy consumption by 70%. The economic analysis shows that these solutions have payback periods of between 10 and 16 years.
2023	Besen et al. [24]	Opaque envelope: Internal wall insulation, insultation of roof and underfloor Transparent envelope: Addition of efficient secondary windows HVAC: Heat recovery ventilation	Experimental and numerical	Several locations of New Zealand	Academic offices, labs etc—1903–1920	The package that implements all the technologies has allowed for a reduction in heating energy demand by up to 90% in all of the climatic locations.
2017	Serraino et al. [25]	Opaque envelope: Roof reconstruction and insulation Transparent envelope: Replacement of window components HVAC: Heat pump, cogeneration, heat recovery ventilation systems	Real case study	Turin (Italy)	San Martino Castle—XIII and extended in the following periods	The interventions determine an improvement in indoor comfort and air quality, and an increase in the building energy efficiency, by reducing emissions as well as containing the running operational costs, all while complying with architectural conservation regulations (to preserve the valuable frescoes).
2016	Mancini et al. [26]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of window components HVAC and management of the building	Numerical	Rome (Italy)	Residential—beginning of 900	The correct management of the opening of the windows, the limitation of the indoor set-point in winter season, the replacement of the window components and the replacement of the heating systems for the toilet zones involve a reduction in energy demand of about 67%.
2015	Şahin et al. [27]	Opaque envelope: Exterior and interior wall, roof and ground floor insulation Transparent envelope: Replacement of windows and weather stripping Lighting: LED lamps HVAC: Heating system and thermostat control	Numerical	İzmir (Turkey)	Hotel—end of the 19th century	The highest energy saving, of around 53%, is achieved by coupling all the interventions. The installation of an air source heat pump alone involves savings of around 25% on the energy demand, while the less effective intervention is the thermal insulation of the ground floor (with savings of only 0.5%).
2020	Piselli et al. [28]	HVAC: Ground Source Heat Pump (GSHP) water-to-water vapor compression	Experimental and numerical	Perugia (Italy)	Medieval complex (new use: office)—second half of XIX century	The proposed intervention results in a reduction in the energy demand for heating by up to 73%. CO2 emissions are also reduced by approximately 69%, and operating costs are significantly lowered.
2015	Franco et al. [29]	Opaque envelope: Thermal insulation of all components Transparent envelope: Addition of secondary windows HVAC: Biogas microturbines (cogeneration) Lighting: LED lamps	Numerical	Genova (Italy)	Albergo dei Poveri (currently abandoned, new use: campus)—1656 and extended in the following periods till 1832	Thermal insulation of walls and floors leads to a reduction in energy demand. The addition of new windows and insulating the roof can determine energy savings of 32%. For lighting needs, a potential 50% reduction is expected with the LED system. The proposed plant intervention results in a significant decrease in CO2 and NOx emissions, along with other greenhouse gases.
2016	L. Mauri [30]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of window glasses HVAC(RES): Geothermal heat pumps Lighting: LED lamps with control RES: PV panels	Numerical analysis	Agrigento (Italy)	Province of Agrigento offices—1860	The intervention allows for a reduction in energy consumption by up to 30% (16.2 kWh/m3). With the photovoltaic technologies, energy consumption would fall below the threshold of 5 kWh/m3 and CO₂ emissions would reach 1 kg/m3, enabling the achievement of the nZEB (Nearly Zero Energy Building) target.
2022	Cho et al. [31]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of window components Lighting: LED lamps RES: PV panels	Numerical	Seoul (Korea)	Dormitory—1922	From an economic point of view, the replacement of the light equipment is the most effective. The application of PV and shading systems have the highest impact on the preservation of the building, according to experts of the field. By selecting the interventions with the least impact on the historical value, the reduction in building energy demand is about 16%.
2013	Moschella et al. [32]	Opaque envelope: Insulation of wall and roof Transparent envelope: Replacement of window glasses RES: PV panels (BIPV efficiency of 5%)	Experimental and numerical	Catania (Italy)	Residential building—not declared	The energy improvement of the building envelope determines a reduction in energy consumption of 19% in winter and 32% in summer. The installation of photovoltaic tiles has contributed to 52% in savings in lighting energy demand.
2023	Jiang et al. [33]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of window glasses HVAC: Heat pump and radiant floor RES: PV opaque and integrated in the building	Experimental and numerical	Trieste (Italy)	Casa Jelinek (various uses)—1860	The internal insulation, combined with the installation of a heat pump, helps to improve both living comfort and the overall energy efficiency of the building. The renewable energy source technologies meet about 45.7% of the requirements. Even without achieving a complete coverage, their adoption is essential to reduce energy purchase from the grid and to promote self-sufficiency.
2020	Cho et al. [34]	Opaque envelope: Internal walls and roof insulation HVAC: Ceiling-type air conditioner Lighting: LED lamps RES: PV panels	Numerical	Seoul (Korea)	Exhibition hall of a university—1924	By coupling passive and active energy retrofit interventions, encompassing the replacement of light equipment with LED, the energy demand for heating and cooling was reduced, respectively, by about 54% and 43%.
2023	Lin et al. [35]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of window components Lighting: LED lamps HVAC RES: PV panels	Numerical	Jiangsu (China)	Several university buildings—1900 and extended in the following years	The energy analysis of a complex of 19 buildings is investigated. The intervention on the active systems, LED and HVAC, and the improvement of the building envelope properties allow for average savings of around 27% for the entire complex. Thanks to the application of the PV system, the savings are higher than 100%, i.e., the energy demand of the building is lower than the energy conversion.
2023	Charalambous et al. [36]	HVAC and RES: Hybrid AC-DC distribution system to connect photovoltaic panels, battery (electrical and thermal (PCM)), reversible heat pump and electrical loads	Experimental and numerical	Aglantzia, (Cyprus)	Vernacular dwelling (event space)—not declared	The proposed system reduced imported energy from the grid and ensured that over 85% of the energy required comes from renewable sources. Moreover, an improvement in thermal comfort by preventing summer overheating and reducing the need for additional heating during winter is guaranteed, leading to a reduction in energy costs.
2024	Park et al. [37]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of window components Lighting: LED lamps HVAC: Fan coil unit RES: PV panels	Numerical	Korea	Educational—1950	The improvement of the HVAC system, the installation of the PV and of LED lights are the most energy-effective interventions. The indoor thermal comfort is improved with the replacement of window components, installation of LED lights, PV panels and the improvement of the HVAC system.
2021	Ozbalta et al. [38]	Opaque envelope: Thermal insulation of all components Transparent envelope: Replacement of window components HVAC: 3-pipe VRV system, heat pumps Lighting: LED lamps RES: PV panels	Experimental and numerical	Izmir (Turkey)	Office—first half of the 19th century	The optimal configuration, internal wall insulation, replacement of windows, LED lighting systems, VRV three-pipe system and the installation of photovoltaic panels, leads to savings of around 34% in energy demand, and a reduction in total costs by 15.71%. The study also highlighted that the individual application of certain technologies results in minimal energy savings compared to the costs incurred.
2023	Gonzalez-Prieto et al. [39]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of window components HVAC: Heat pumps RES: PV panels	Numerical	Gijon (Spain)	Office—1910 renovated in the following years	Envelope interventions have reduced the building’s energy consumption from 59.22 to 43.05 kWh/m2⋅year. Passive interventions, despite longer payback times compared to active systems, prove to be more sustainable by reducing the environmental impact and offering energy savings of up to seven times greater than the impact of the materials used.
2017	Rodrigues et al. [40]	Opaque envelope: Interior wall, roof insulation Transparent envelope: Replacement of windows HVAC: High-efficiency air conditioning system	Numerical	Coimbra (Portugal)	Residential and office—beginning of the 20th century	The energy effectiveness of the interventions is investigated with respect to the intended use and the occupancy of the building. Moreover, the life cycle impact is investigated. The highest life cycle impact for thermal insulation is achieved in a residential environment with a high employment rate.
2016	Tiberi et al. [41]	Opaque envelope: Thermal insulation of all components Transparent envelope: Replacement of window components HVAC: Cogeneration system and thermostatic valves RES: PV panels	Numerical	Rome (Italy)	University “La Sapienza”—1962	The most energy and environmentally efficient scenario involves a complete upgrade of the building envelope, the installation of a photovoltaic system and the use of thermostatic valves. This combination reduces the heating demand by 91%, achieving overall energy savings of 42% and a 39% reduction in CO₂ emissions. However, this scenario has a payback period of over 30 years.
2015	Bellia et al. [42]	Opaque envelope: Insulation of basement Transparent envelope: Replacement of window glasses and frames HVAC: Installation of a radiant panel heating system RES: PV roofing	Experimental and numerical	Naples (Italy)	Real Albergo dei Poveri (currently abandoned, new use: youth city)—second half of the XVIII century	The intervention on the building envelope ensures energy savings of 27.1% in the winter season. The installation of opaque roofing was the best choice for the use of renewable energy, increasing energy conversion by 20% compared to semi-transparent roofing and reducing the risks of glare and summer overheating. Overall, the interventions reduce CO2 emissions by 32.1 tons/year.
2015	Dalla Mora et al. [43]	Opaque envelope: Insulation of internal walls and roof Transparent envelope: Replacement of window components HVAC: Reversible HP, radiant floor system and MVHR RES: Solar thermal panels and PV panels	Experimental and numerical	Treviso (Italy)	Ca’ S. Orsola (residential building)—not declared	Thanks to the interventions, the energy consumption was reduced by 92.5% while greenhouse gas emissions decreased by 81%, transforming the building into an nZEB (Nearly Zero Energy Building). The investment proved economically advantageous, with an estimated payback period of 13 years.
2020	Sugár et al. [44]	Opaque envelope: Thermal insulation of all components Transparent envelope: Replacement of window components HVAC: District heating reversible heat pump and condensation heater	Numerical	Budapest (Hungary)	District—at the turn of 19th–20th century	The optimal solutions involve applying the “Least Invasive” scenario for traditional historic buildings and the “Nearly Zero” scenario for those built after World War II. Combining these scenarios with a heat pump allow for 69% energy savings for heating and domestic hot water production. In total, 85% of the buildings meet the Nearly Zero Energy Building (nZEB) requirements, compared to the initial 4.7%.
2018	Belpoliti et al. [45]	Opaque envelope: Insulation of all components Transparent envelope: Replacement of window components HVAC: Condensing boiler and thermostatic valves	Numerical	Aquila (Italy)	Coperciano town—before 1950 for the historical buildings	With the current occupancy at 36%, the energy consumption after the retrofitting would be 2.33 GWh/year. If repopulation reaches 60%, the energy demand will increase to 3.77 GWh/year, which is still an improvement compared to the current 4.91 GWh/year under the semi-abandoned condition.
2022	Teso et al. [46]	Opaque envelope: Internal and external wall insulation and roof insulation Transparent envelope: Replacement of window glasses HVAC: Condensing boiler	Numerical	Venice (Italy)	District with multi-family buildings—1920–1928	The combination of retrofit measures achieved energy savings of 67% at the district level, reducing CO2 emissions by 1.1 million tons per year and decreasing air pollution. All of this was accomplished while respecting the cultural and architectural heritage constraints.
2022	Dehwah et al. [47]	Opaque envelope: Interior wall and roof insulation Lighting: LED lamps HVAC: High-efficiency air conditioning system RES: PV panels	Numerical	Al-Baha (Saudi Arabia)	Ancient village transformed into a hotel complex—8th century	To avoid the high energy demand, and to meet the well-being of the occupants, when transforming the village into a hotel, the installation of LED lights, the thermal insulation of walls and roof and the installation of an efficient system for the microclimatic control is required. The installation of PV systems off-site will lead to the hotel meeting the target of Net Zero Energy Buildings.
2024	Tajalli-Ardekani et al. [48]	Envelope: Improvement of building envelope HVAC: Trigeneration systems (cogeneration with a microturbine coupled to an absorption chiller) Lighting: LED lamps RES: PV panels and battery	Experimental (digital twin) and numerical	Rome (Italy)	District—not declared	The proposed solutions, managed by an intelligent control system, have led to a reduction of about 55% in primary energy demand and of about 56% in CO₂ emissions. The measures also involved a 68.2% decrease in energy purchased from the grid and a 48.9% reduction in energy generation costs. However, the payback period was estimated at 20 years, which is not optimal.
2019	Blázquez et al. [49]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of window components HVAC: Mechanical ventilation and MHVR	Experimental and numerical	Seville (Spain)	Residential complex—1950–1963	The improvement of building envelope components leads to a high reduction in energy demand (up to around 28%). Mechanical ventilation ensures better air quality, while heat recovery prevents an increase in winter energy demand.
2021	Elnagar et al. [52]	Opaque envelope: Thermal insulation of walls, roof and basement HVAC: Heat recovery ventilator and geothermal system RES: PV and thermal solar collector	Numerical	Vienna (Austria)	Student residence—1907	Rigid cellulose board and VIP panels were implemented to improve the building envelope performance. Moreover, a well-designed PV system, respecting the historical value of the building, could satisfy around 22% of the annual energy demand. The coupling of the proposed solutions will allow the building to reach the passive house standard in Austria.
2021	Coelho et al. [53]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of windows	Numerical	Lisbona (Portugal)	Church—13th century	In this study, it is highlighted that the improvement of opaque and transparent building envelope components can improve the energy performance of the building across current and future weather conditions. However, high care should be applied in the selection of the intervention in order not to affect the historical value of the artifacts.

Table 1. Reviewed studies: investigated technology, type of investigation, location, type of building and main findings.

Year	Authors	Technology	Type of Investigation	City	Type of Building—Year of Construction	Main Findings and Results
2023	Aparicio-Fernandez et al. [6]	Opaque envelope: Thermal insulation of all components Transparent envelope: Replacement of window components	Experimental and numerical	Valencia (Spain)	Music school—1921	Energy demand can be reduced by 55.2%, reaching the Passivhaus standard. Heating demand decreases; however, it will be necessary to implement increased natural ventilation in the summer season to contain the rise in energy demand for cooling.
2015	Ascione et al. [7]	Opaque envelope: Internal wall insulation and insulation of roof Transparent envelope: Replacement of window components HVAC: Zonal thermostats	Experimental and numerical	Benevento (Italy)	Administrative and didactic buildings—1927	The proposed scenario leads to a 38% reduction in the annual primary energy demand and in greenhouse gas emissions. The payback period is around 15 years. Furthermore, the optimal management of microclimate control systems resulted in electricity savings of 24%.
2019	Galatioto et al. [8]	Opaque envelope: Internal insulation of walls and roof Transparent envelope: Replacement of window glasses and shading devices Lighting: LED lamps and lighting control	Numerical	Cagliari, Roma, Milano, Palermo (Italy)	Office—1938	The most effective intervention consists of internal thermal insulation, roof insulation and the replacement of windows with shading devices, involving energy savings of about 51% in Palermo, 52% in Cagliari, 56% in Rome and 43% in Milan. The chosen scenario allows the highest reductions in CO2. However, the payback time is over 30 years.
2021	Qu et al. [9]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of existing components	Numerical	Nottingham (UK)	Victorian-style house—1884	The optimal cost–benefit solution leads to a 51.8% reduction in primary energy demand with a payback period of around 18 years. While winter comfort improves, summer temperatures rise by 2.6 °C, so adequate ventilation is recommended.
2024	Yuk et al. [10]	Transparent envelope: Application of secondary windows	Numerical	Seoul (Korea)	University office—1924	The adopted solution involves a saving of around 17% and 24% in cooling and heating energy demand, and determines more stable conditions in terms of indoor air temperature and relative humidity.
2017	Lucchi et al. [11]	Opaque envelope: Internal wall insulation	Numerical	Como (Italy)	Residential villa—XVIII Century	All the proposed materials are energy-effective as internal insulation materials, while economic convenience is not always achieved, especially with the greater thicknesses of cork, perlite and foam glass. Mineral glass wool emerged as the most cost-effective material. With the intervention, the building is close to Nearly Zero Energy Building (nZEB) standards.
2015	Tadeu et al. [12]	Opaque envelope: External wall insulation, roof and floor insulation Transparent envelope: Replacement of window components HVAC: Substitution of heating and domestic hot water systems	Numerical	Coimbra (Portugal) and other Portugal localities	Commercial and residential—not declared	The main results concerned the thickness of the insulating materials. Therefore, the optimal thickness changed according to the Portugal localities, which differ for the heating degree days value. A higher thickness is required in cities with higher HDD. Moreover, an LCA analysis is conducted, and it revealed a balance between the embodied and operational impacts.
2016	Cornaro et al. [13]	Opaque envelope: Insulation of external walls and basement Transparent envelope: Replacement of window components	Experimental and numerical	Rome (Italy)	Villa Mondragone (conference center and foresteria)—1573	The replacement of the windows reduced the energy demand from 116.9 kWh/m2 to 68.2 kWh/m2. Subsequently, the insulation of the external walls and floors further decreased the heating energy consumption to 42.2 kWh/m2.
2017	Roberti et al. [14]	Opaque envelope: Internal and external walls and roof insulation Transparent envelope: Replacement of window components Natural ventilation	Numerical	Bolzano (North Italy)	Medieval building transformed in museum—end of the twelfth century	The optimal energy retrofit solutions are identified through a balance between the energy efficiency and the preservation of the building. The last is identified through a survey of the experts of heritage conservation. The results highlight that the refurbishment involves high energy savings, but further improvements will lead to a degradation of the building.
2020	Cho et al. [15]	Opaque envelope: Insulation of internal walls and roof Transparent envelope: Replacement of window glasses and internal blinds Lighting: LED lamps	Experimental and numerical	Seoul (Korea)	Museum and office (Taehan Medical Center)—1907	The package integrating all proposed technologies has proven highly effective, reducing the heating energy demand by up to 72%. The insulation of the walls and roof has drastically minimized the risk of mold, bringing the mold index down to very low, thereby enhancing the safety and preservation of the displayed collections.
2018	Zazzini et al. [16]	Opaque envelope: Installation of a green wall at the back of the library	Numerical	Foggia (Italy)	Library—1858	The intervention involves a reduction of approximately 17.8% in electricity demand and 28% in gas usage, with a positive impact on the residents’ comfort as well. The payback period has been estimated to be around 7–8 years.
2017	D’Agostino et al. [17]	HVAC: Condensing boiler, heat pump, chiller, reversible heat pump, packaged air conditioning units	Numerical	Naples (Italy)	Paleontology museum—between 5th and 7th century	The most energy-efficient solution is the use of a reversible heat pump that allows for the guarantee of an acceptable thermal comfort in the summer period as well, resulting in a primary energy demand of approximately 97 kWh/m2, with only a minimal increase of 5% compared to the current consumption (which does not include cooling).
2010	Camuffo et al. [18]	HVAC: Substitution of the heating system	Monitoring	Rocca Pietore and Santo Stefano di Cadore (Italy)	Church—15th century	A proper distribution of the heated air is required in order to guarantee acceptable conditions of air temperature and relative humidity for the occupants, avoiding a deterioration of the artifacts and paintings, and therefore, the accumulation of hot air in these areas.
2024	Etemad et al. [19]	HVAC: Air cooled chiller, variable refrigerant flow, split DX systems	Numerical	Tehran (Iran)	Museum—1933	The VRF system is the most effective with the lowest annual cooling electricity demand, with a reduction of around 50% with respect to the current situation.
2023	Pochwała et al. [20]	HVAC: Six LPG-powered air-source heat pumps and five gas condensing boilers, four electric-powered air compressor heat pumps and one electric induction boiler	Numerical	Woskowice Małe (Poland)	Rehab treatment facility—1800, reconstructed in 1912	The adoption of four electric heat pumps is preferable to that of six LPG heat pumps. This advantage is due to a greater reduction in heating energy costs (72% compared to 59%) and overall energy savings exceeding 70%. Both scenarios are environmentally friendly, ensuring a reduction in CO₂ emissions of around 121.1 tons.
2015	Ascione et al. [21]	Opaque envelope: Internal/external wall insulation and insulation of roof Transparent envelope: Replacement of window components HVAC: Several types of boilers, chiller (air-cooled), several types of heat pumps	Experimental and numerical	Naples (Italy)	Palazzo Penne (office)—1406	The optimal package consists of installing fan-coil units powered by a condensing boiler and a high-efficiency air chiller, combined with roof insulation and the adoption of selective or low-emissivity glass. This approach ensures energy and environmental savings of 38% and reduces overall costs by approximately 20%.
2022	Milovanović et al. [22]	Opaque envelope: External wall insulation Transparent envelope: New secondary windows, old existing windows (repaired and sealed) HVAC: Heat recovery ventilation system, condensing boiler	Numerical	Zagreb (Croatia)	Multi-residential building—1920	The results show that interventions on the building envelope can reduce primary energy consumption by up to 53%. Including mechanical ventilation and improving airtightness, the reduction can reach up to 67%. By also replacing the technical system with a more efficient one, the energy demand and emissions can be reduced by up to 75%.
2016	Blecich et al. [23]	Opaque envelope: Thermal insulation of opaque components Transparent envelope: Replacement of window components External shading HVAC: MVHR	Numerical	Istria (Croatia)	Castle of Krsan (currently abandoned, new use: tourist residences)—1274, subjected to several changes	Envelope insulation, efficient windows, improved airtightness and the MVHR system can reduce heating demand by over 80%. Strategies, such as external shading and night ventilation, can reduce cooling energy consumption by 70%. The economic analysis shows that these solutions have payback periods of between 10 and 16 years.
2023	Besen et al. [24]	Opaque envelope: Internal wall insulation, insultation of roof and underfloor Transparent envelope: Addition of efficient secondary windows HVAC: Heat recovery ventilation	Experimental and numerical	Several locations of New Zealand	Academic offices, labs etc—1903–1920	The package that implements all the technologies has allowed for a reduction in heating energy demand by up to 90% in all of the climatic locations.
2017	Serraino et al. [25]	Opaque envelope: Roof reconstruction and insulation Transparent envelope: Replacement of window components HVAC: Heat pump, cogeneration, heat recovery ventilation systems	Real case study	Turin (Italy)	San Martino Castle—XIII and extended in the following periods	The interventions determine an improvement in indoor comfort and air quality, and an increase in the building energy efficiency, by reducing emissions as well as containing the running operational costs, all while complying with architectural conservation regulations (to preserve the valuable frescoes).
2016	Mancini et al. [26]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of window components HVAC and management of the building	Numerical	Rome (Italy)	Residential—beginning of 900	The correct management of the opening of the windows, the limitation of the indoor set-point in winter season, the replacement of the window components and the replacement of the heating systems for the toilet zones involve a reduction in energy demand of about 67%.
2015	Şahin et al. [27]	Opaque envelope: Exterior and interior wall, roof and ground floor insulation Transparent envelope: Replacement of windows and weather stripping Lighting: LED lamps HVAC: Heating system and thermostat control	Numerical	İzmir (Turkey)	Hotel—end of the 19th century	The highest energy saving, of around 53%, is achieved by coupling all the interventions. The installation of an air source heat pump alone involves savings of around 25% on the energy demand, while the less effective intervention is the thermal insulation of the ground floor (with savings of only 0.5%).
2020	Piselli et al. [28]	HVAC: Ground Source Heat Pump (GSHP) water-to-water vapor compression	Experimental and numerical	Perugia (Italy)	Medieval complex (new use: office)—second half of XIX century	The proposed intervention results in a reduction in the energy demand for heating by up to 73%. CO2 emissions are also reduced by approximately 69%, and operating costs are significantly lowered.
2015	Franco et al. [29]	Opaque envelope: Thermal insulation of all components Transparent envelope: Addition of secondary windows HVAC: Biogas microturbines (cogeneration) Lighting: LED lamps	Numerical	Genova (Italy)	Albergo dei Poveri (currently abandoned, new use: campus)—1656 and extended in the following periods till 1832	Thermal insulation of walls and floors leads to a reduction in energy demand. The addition of new windows and insulating the roof can determine energy savings of 32%. For lighting needs, a potential 50% reduction is expected with the LED system. The proposed plant intervention results in a significant decrease in CO2 and NOx emissions, along with other greenhouse gases.
2016	L. Mauri [30]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of window glasses HVAC(RES): Geothermal heat pumps Lighting: LED lamps with control RES: PV panels	Numerical analysis	Agrigento (Italy)	Province of Agrigento offices—1860	The intervention allows for a reduction in energy consumption by up to 30% (16.2 kWh/m3). With the photovoltaic technologies, energy consumption would fall below the threshold of 5 kWh/m3 and CO₂ emissions would reach 1 kg/m3, enabling the achievement of the nZEB (Nearly Zero Energy Building) target.
2022	Cho et al. [31]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of window components Lighting: LED lamps RES: PV panels	Numerical	Seoul (Korea)	Dormitory—1922	From an economic point of view, the replacement of the light equipment is the most effective. The application of PV and shading systems have the highest impact on the preservation of the building, according to experts of the field. By selecting the interventions with the least impact on the historical value, the reduction in building energy demand is about 16%.
2013	Moschella et al. [32]	Opaque envelope: Insulation of wall and roof Transparent envelope: Replacement of window glasses RES: PV panels (BIPV efficiency of 5%)	Experimental and numerical	Catania (Italy)	Residential building—not declared	The energy improvement of the building envelope determines a reduction in energy consumption of 19% in winter and 32% in summer. The installation of photovoltaic tiles has contributed to 52% in savings in lighting energy demand.
2023	Jiang et al. [33]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of window glasses HVAC: Heat pump and radiant floor RES: PV opaque and integrated in the building	Experimental and numerical	Trieste (Italy)	Casa Jelinek (various uses)—1860	The internal insulation, combined with the installation of a heat pump, helps to improve both living comfort and the overall energy efficiency of the building. The renewable energy source technologies meet about 45.7% of the requirements. Even without achieving a complete coverage, their adoption is essential to reduce energy purchase from the grid and to promote self-sufficiency.
2020	Cho et al. [34]	Opaque envelope: Internal walls and roof insulation HVAC: Ceiling-type air conditioner Lighting: LED lamps RES: PV panels	Numerical	Seoul (Korea)	Exhibition hall of a university—1924	By coupling passive and active energy retrofit interventions, encompassing the replacement of light equipment with LED, the energy demand for heating and cooling was reduced, respectively, by about 54% and 43%.
2023	Lin et al. [35]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of window components Lighting: LED lamps HVAC RES: PV panels	Numerical	Jiangsu (China)	Several university buildings—1900 and extended in the following years	The energy analysis of a complex of 19 buildings is investigated. The intervention on the active systems, LED and HVAC, and the improvement of the building envelope properties allow for average savings of around 27% for the entire complex. Thanks to the application of the PV system, the savings are higher than 100%, i.e., the energy demand of the building is lower than the energy conversion.
2023	Charalambous et al. [36]	HVAC and RES: Hybrid AC-DC distribution system to connect photovoltaic panels, battery (electrical and thermal (PCM)), reversible heat pump and electrical loads	Experimental and numerical	Aglantzia, (Cyprus)	Vernacular dwelling (event space)—not declared	The proposed system reduced imported energy from the grid and ensured that over 85% of the energy required comes from renewable sources. Moreover, an improvement in thermal comfort by preventing summer overheating and reducing the need for additional heating during winter is guaranteed, leading to a reduction in energy costs.
2024	Park et al. [37]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of window components Lighting: LED lamps HVAC: Fan coil unit RES: PV panels	Numerical	Korea	Educational—1950	The improvement of the HVAC system, the installation of the PV and of LED lights are the most energy-effective interventions. The indoor thermal comfort is improved with the replacement of window components, installation of LED lights, PV panels and the improvement of the HVAC system.
2021	Ozbalta et al. [38]	Opaque envelope: Thermal insulation of all components Transparent envelope: Replacement of window components HVAC: 3-pipe VRV system, heat pumps Lighting: LED lamps RES: PV panels	Experimental and numerical	Izmir (Turkey)	Office—first half of the 19th century	The optimal configuration, internal wall insulation, replacement of windows, LED lighting systems, VRV three-pipe system and the installation of photovoltaic panels, leads to savings of around 34% in energy demand, and a reduction in total costs by 15.71%. The study also highlighted that the individual application of certain technologies results in minimal energy savings compared to the costs incurred.
2023	Gonzalez-Prieto et al. [39]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of window components HVAC: Heat pumps RES: PV panels	Numerical	Gijon (Spain)	Office—1910 renovated in the following years	Envelope interventions have reduced the building’s energy consumption from 59.22 to 43.05 kWh/m2⋅year. Passive interventions, despite longer payback times compared to active systems, prove to be more sustainable by reducing the environmental impact and offering energy savings of up to seven times greater than the impact of the materials used.
2017	Rodrigues et al. [40]	Opaque envelope: Interior wall, roof insulation Transparent envelope: Replacement of windows HVAC: High-efficiency air conditioning system	Numerical	Coimbra (Portugal)	Residential and office—beginning of the 20th century	The energy effectiveness of the interventions is investigated with respect to the intended use and the occupancy of the building. Moreover, the life cycle impact is investigated. The highest life cycle impact for thermal insulation is achieved in a residential environment with a high employment rate.
2016	Tiberi et al. [41]	Opaque envelope: Thermal insulation of all components Transparent envelope: Replacement of window components HVAC: Cogeneration system and thermostatic valves RES: PV panels	Numerical	Rome (Italy)	University “La Sapienza”—1962	The most energy and environmentally efficient scenario involves a complete upgrade of the building envelope, the installation of a photovoltaic system and the use of thermostatic valves. This combination reduces the heating demand by 91%, achieving overall energy savings of 42% and a 39% reduction in CO₂ emissions. However, this scenario has a payback period of over 30 years.
2015	Bellia et al. [42]	Opaque envelope: Insulation of basement Transparent envelope: Replacement of window glasses and frames HVAC: Installation of a radiant panel heating system RES: PV roofing	Experimental and numerical	Naples (Italy)	Real Albergo dei Poveri (currently abandoned, new use: youth city)—second half of the XVIII century	The intervention on the building envelope ensures energy savings of 27.1% in the winter season. The installation of opaque roofing was the best choice for the use of renewable energy, increasing energy conversion by 20% compared to semi-transparent roofing and reducing the risks of glare and summer overheating. Overall, the interventions reduce CO2 emissions by 32.1 tons/year.
2015	Dalla Mora et al. [43]	Opaque envelope: Insulation of internal walls and roof Transparent envelope: Replacement of window components HVAC: Reversible HP, radiant floor system and MVHR RES: Solar thermal panels and PV panels	Experimental and numerical	Treviso (Italy)	Ca’ S. Orsola (residential building)—not declared	Thanks to the interventions, the energy consumption was reduced by 92.5% while greenhouse gas emissions decreased by 81%, transforming the building into an nZEB (Nearly Zero Energy Building). The investment proved economically advantageous, with an estimated payback period of 13 years.
2020	Sugár et al. [44]	Opaque envelope: Thermal insulation of all components Transparent envelope: Replacement of window components HVAC: District heating reversible heat pump and condensation heater	Numerical	Budapest (Hungary)	District—at the turn of 19th–20th century	The optimal solutions involve applying the “Least Invasive” scenario for traditional historic buildings and the “Nearly Zero” scenario for those built after World War II. Combining these scenarios with a heat pump allow for 69% energy savings for heating and domestic hot water production. In total, 85% of the buildings meet the Nearly Zero Energy Building (nZEB) requirements, compared to the initial 4.7%.
2018	Belpoliti et al. [45]	Opaque envelope: Insulation of all components Transparent envelope: Replacement of window components HVAC: Condensing boiler and thermostatic valves	Numerical	Aquila (Italy)	Coperciano town—before 1950 for the historical buildings	With the current occupancy at 36%, the energy consumption after the retrofitting would be 2.33 GWh/year. If repopulation reaches 60%, the energy demand will increase to 3.77 GWh/year, which is still an improvement compared to the current 4.91 GWh/year under the semi-abandoned condition.
2022	Teso et al. [46]	Opaque envelope: Internal and external wall insulation and roof insulation Transparent envelope: Replacement of window glasses HVAC: Condensing boiler	Numerical	Venice (Italy)	District with multi-family buildings—1920–1928	The combination of retrofit measures achieved energy savings of 67% at the district level, reducing CO2 emissions by 1.1 million tons per year and decreasing air pollution. All of this was accomplished while respecting the cultural and architectural heritage constraints.
2022	Dehwah et al. [47]	Opaque envelope: Interior wall and roof insulation Lighting: LED lamps HVAC: High-efficiency air conditioning system RES: PV panels	Numerical	Al-Baha (Saudi Arabia)	Ancient village transformed into a hotel complex—8th century	To avoid the high energy demand, and to meet the well-being of the occupants, when transforming the village into a hotel, the installation of LED lights, the thermal insulation of walls and roof and the installation of an efficient system for the microclimatic control is required. The installation of PV systems off-site will lead to the hotel meeting the target of Net Zero Energy Buildings.
2024	Tajalli-Ardekani et al. [48]	Envelope: Improvement of building envelope HVAC: Trigeneration systems (cogeneration with a microturbine coupled to an absorption chiller) Lighting: LED lamps RES: PV panels and battery	Experimental (digital twin) and numerical	Rome (Italy)	District—not declared	The proposed solutions, managed by an intelligent control system, have led to a reduction of about 55% in primary energy demand and of about 56% in CO₂ emissions. The measures also involved a 68.2% decrease in energy purchased from the grid and a 48.9% reduction in energy generation costs. However, the payback period was estimated at 20 years, which is not optimal.
2019	Blázquez et al. [49]	Opaque envelope: Internal wall insulation Transparent envelope: Replacement of window components HVAC: Mechanical ventilation and MHVR	Experimental and numerical	Seville (Spain)	Residential complex—1950–1963	The improvement of building envelope components leads to a high reduction in energy demand (up to around 28%). Mechanical ventilation ensures better air quality, while heat recovery prevents an increase in winter energy demand.
2021	Elnagar et al. [52]	Opaque envelope: Thermal insulation of walls, roof and basement HVAC: Heat recovery ventilator and geothermal system RES: PV and thermal solar collector	Numerical	Vienna (Austria)	Student residence—1907	Rigid cellulose board and VIP panels were implemented to improve the building envelope performance. Moreover, a well-designed PV system, respecting the historical value of the building, could satisfy around 22% of the annual energy demand. The coupling of the proposed solutions will allow the building to reach the passive house standard in Austria.
2021	Coelho et al. [53]	Opaque envelope: Interior wall and roof insulation Transparent envelope: Replacement of windows	Numerical	Lisbona (Portugal)	Church—13th century	In this study, it is highlighted that the improvement of opaque and transparent building envelope components can improve the energy performance of the building across current and future weather conditions. However, high care should be applied in the selection of the intervention in order not to affect the historical value of the artifacts.

The investigated interventions, presented in Section 3 and discussed to this point, are collected in Figure 5. The main materials and technologies examined in the literature are classified on the basis of the three main levels of energy efficiency, i.e., building envelope, systems and renewable energy sources.

Regarding the opaque building envelope, the interventions are those commonly adopted in the building sector in general. These include external and internal wall and roof insulation, and the thermal insulation of the floor and basement. The technique involves adding a suitable material, characterized by specific thermo-physical properties (thermal conductivity, density and specific heat). In the case of historic buildings, it is crucial, as previously discussed, to assess the compatibility of the selected material with the existing structures and its performance in terms of moisture management. This is essential to prevent damage to internal frescoes and paintings and, especially, to ensure proper indoor thermal conditions for the occupants. As concerns the transparent building envelope, the main widespread solution consists of the replacement of the window components with more efficient ones (lower thermal transmittance), such as double glasses and double low-emissivity ones. The intervention about the frame of the transparent component is less common, as discussed in Figure 3.

For the technical systems, the interventions lie in the substitution of the heating and/or cooling systems or the installation of the components where not present. An important consideration is regarding the adoption of proper systems for the ventilation inside the environment. As regards the integration of renewable energy sources, the topic is extremely critical for historic buildings. However, the necessity to integrate renewable energies to reach high energy standards, such as zero energy building, was highlighted in several studies.

In addition to the evidence of the energy efficiency measures available in the scientific literature, another detail that emerged from Figure 5 is the lack of adoption and investigation of certain measures. Concerning the opaque building envelope, the adoption of more innovative materials, such as phase change materials, living walls, cool roof materials, or technologies, like the green roof, is not proposed. With regard to the transparent building envelope, research about the adoption of more innovative materials, such as selective, solar-controlled, thermochromic glasses, could be explored in future studies. A further technology worthy of investigation could be the application of transparent double skin façades coupled with a structural intervention, i.e., an exoskeleton. This solution also has been investigated in the literature with reference to historic buildings [53,54], and it can be easily matched with a double skin façade, a second completely transparent external layer. As always, in the case of historical buildings, and with buildings in general, careful design has to be conducted. So, the feasibility should be assessed with a tailored approach with respect to the investigated building.

Technologies for micro-climate control, such as radiant panels, or the exploitation of renewable energy sources, such as wind, are also not addressed. An accurate analysis about the compatibility of the energy efficiency measures is, in any case, required.

The main themes addressed in the presented review study, and therefore, in the discussed papers, are summarized in Figure 6, which collates all the “keywords” identified by the authors of each reviewed paper.

The topic of the historic building sector is obviously central, together with that of its energy efficiency through the application of several energy retrofit interventions. It is possible to understand that the studies investigated several aspects, such as economic, environmental and thermal comfort, beyond the purely energetic one. Some criticalities can be discovered, such as the reduction in indoor volume, the attention to the conservation, and therefore, the preservation of the buildings (their artistic/historic value). Several solutions are investigated in the scientific literature to overcome these limitations and to achieve an enhancement in the energy performance of the building while guaranteeing the preservation of the building. For example, concerning the indoor available space, the adoption of VIP panels for thermal insulation is investigated, resulting in very effective strategies [8,9,52] which were able to guarantee good thermal properties for the building envelope components with lower thicknesses. The implementation of renewable energy sources, in particular PV panels and shading systems, is identified as the most impactful intervention on the architectural and historical worth of the buildings.

However, what emerged from Figure 6, which collates the “keywords” declared by the authors of the reviewed papers, is that the scope of the energy retrofit intervention, or even the main limitations, cannot be clearly identified from these keywords. For this reason, a further figure has been realized, in a similar form, i.e., a “word cloud” (Figure 7). In this second case, the cloud is realized by deeply examining all the reviewed papers, identifying, for each one, the levels of the applied energy efficiency interventions, i.e., building envelope, plant systems and renewable energy sources, and the specific intervention investigated (the ones that have been detailed in Section 3 and Section 4). In summary, new “keywords” have been assigned to each reviewed paper by collecting the investigated technologies for energy improvement in the historical building sector. Furthermore, the types of analysis and approaches to the study were also collected.

In this case, the central topic is still “historic building” and its energy analysis and improvement. It is possible to understand that there is a predominance of numerical (simulative) studies over experimental ones and that energy analysis is the most common with respect to economic, environmental or thermal comfort. The economic and environmental analyses have a similar frequency. Concerning the thermal comfort analysis, it was properly conducted, through the assessment of the main indicators PMV and PPD, the discomfort hours in both heating and cooling seasons, or through the evaluation of the occupant’s satisfaction with respect to the indoor conditions, in very few studies. Overall, achieving the right balance and making the proper selection and combination of retrofit technologies should consider all the factors; therefore, paying greater attention to the topics less investigated, such as maintaining optimal indoor thermal comfort, is suggested for future research. This involves consistent temperature regulation, humidity control and air quality to create a comfortable and healthy indoor environment and the evaluation of how the proposed solutions can contribute to thermal comfort by reducing the discomfort hours. The research surrounding the approaches that are able to promote the sustainable improvement of the historic buildings by enhancing thermal comfort is evenly conducted among the literature [55].

Among the interventions for the building envelope, a predominance of indoor applications for insulating materials compared to external ones is observed, and a similar frequency concerning the replacement of transparent components is shown. This is mainly related to the impact on both heating and cooling seasons that these systems can determine, and thus the necessity to find the right balance between the two main energy needs exists. Technical solutions, such as geothermal systems or lighting optimization, are discussed less frequently, and only one-third of the studies explore the installation of photovoltaic systems, underscoring the challenges of integrating this technology into historic buildings.

6. Conclusions

The energy requalification of historic buildings in Europe cannot be overlooked, considering the high percentage of existing heritage architectures and the objectives of achieving climate neutrality by 2050. The design phase and the selection of possible energy retrofit measures (materials or technologies) for such types of buildings are particularly challenging because there are stringent requirements to preserve the artistic, architectural, cultural and historical values of these buildings while simultaneously improving their energy performance. Such aspects require—or better, should require, although this was not evident in the reviewed studies—a collaboration among experts from different sectors, such as architects, engineers and specialists in cultural heritage conservation. Therefore, this is the first underestimated aspect in the existing literature that should be considered in future studies (interdisciplinarity collaboration). In the following section, the main findings and limitations of the review study presented are reported, and we suggest aspects and directions for further research in the field of energy upgrading of historic buildings.

From the reviewed studies, a clear dependence of the effectiveness of the interventions on the climate location, and therefore, of the typical energy needs that characterize a specific locality, has emerged. The application of thermal insulation for the vertical walls or the replacement of the windows with more efficient ones (by also reducing the infiltration through the frame components) do not have a unique effect on the energy behavior of the building, and above all, on the management of humidity. Therefore, it is necessary to carefully design the interventions, for instance, by applying the thermal insulation at a selected number of building façades or differentiating the window selection based on their exposure, thus avoiding summer overheating while guaranteeing a proper solar heat gain in winter. Last but not least, under a scenario of rapidly changing climates, paying special attention to mitigation strategies to avoid overheating would also be advisable [56]. In addition, the survey on the energy performance of buildings under future climatic conditions, according to the projections provided by the Intergovernmental Panel on Climate Change, is increasingly being investigated, and its wide spread across the historic building sector is advisable as a future step in the research.

Focusing on the insulating materials, it is fundamental to not only satisfy requirements in terms of thermal insulation (e.g., thermal conductivity) but also to guarantee a proper compatibility with the existing structure and materials that make up the building. For example, attention should be focused on the breathability of the insulating material to prevent issues like condensation or mold, which are especially problematic in buildings with frescoes and paintings that need to be protected and preserved.

The materials used in the historical structures are typically able to guarantee proper breathability, which helps in the control of moisture levels, avoiding the risk of condensation or mold, as also shown in Figure 2. When designing an intervention to the opaque building envelope, this aspect should be carefully considered. The addition of insulating material, if not properly chosen in terms of thermo-physical properties and thickness, can involve condensation and moisture issues rather than a reduction in the breathability. The ability of the building to regulate moisture and temperature levels highly affect the health of the occupants. Therefore, an excessively insulating hyper-insulation, without adequate considerations on moisture management, should be avoided. In the revised studies, the topic of breathability is not adequately addressed; it is only briefly mentioned in a few studies, when introducing materials [57] or simply in the conclusions as an important aspect to consider in order to evaluate the compatibility of materials with historic structures [58,59]. Therefore, paying more attention to this theme is recommended.

Considerations or analyses about the formation of mold and moisture-related risks are revealed to be largely underexplored in the reviewed literature. In fact, only four studies which addressed the problems of moisture management, condensation and mold formation and identified insulating materials that could better address these problems have been found. However, this is a well-known problem discussed in the literature [60,61] that does not propose the energy, structural or other type of redevelopment of historic buildings. There are studies that analyze the problem of humidity and condensation [62,63,64] but they do not always suggest solutions to solve them and, especially if an intervention is proposed, it is not investigated from the energy point of view. Therefore, there is a decoupling between the studies that investigate and propose retrofit solutions to improve the energy and hygrothermal behavior of historic buildings. On the other hand, proper ventilation can help to regulate this phenomenon, and a large part of the study proposed the installation, substitution or improvement of the ventilation system.

And yet, another aspect to consider is the reversibility of the interventions, ensuring the restoration of initial conditions without permanently damaging the basic structure, thus enabling the easy removal of the materials. The selected materials for application on historic buildings should be durable to minimize the frequency of interventions, and therefore, always protect the integrity of the property.

A further aspect that has not been sufficiently investigated concerns the biodegradability and recyclability of the materials, which could be analyzed and assessed through a life cycle assessment (LCA) approach. The latter is becoming increasingly widespread in recent years in the construction field and deserves to be explored with reference to historic buildings.

It has emerged that attention is mainly paid to the energy and environmental aspects, while the investigation of indoor thermal comfort is limited. This aspect is crucial and should be taken into account, firstly, because the energy demand of a building in real operation is strictly dependent on the occupant’s satisfaction, and secondly, because with reference to historical buildings hosting frescoes and pictures, there are stringent requirements to maintain adequate conditions for the conservation of the cultural artifacts.

Finally, when examining the broader topic of building energy retrofitting, it emerged that several practices and advanced methods commonly employed in the literature are not yet applied in the context of historic buildings. For instance, analyses using computational fluid dynamics methods, which allows for the exploration of the 3D distribution of microclimatic parameters, and therefore, the evaluation of the indoor air quality, are underutilized.

Similarly, techniques such as artificial neural networks and pareto optimization have not been widely implemented. These methods are particularly effective to easily process and compare a large number of energy interventions, and to identify the best trade-off between multiple objectives. In future research, it is essential to explore how these techniques can be tailored for application in heritage conservation. This could offer a pathway to achieve sustainability goals without altering the cultural heritage.

In conclusion, the improvement of the energy performance of historic buildings is a complex and critical challenge for reaching sustainability goals, necessitating a multidisciplinary approach and innovative, compatible solutions to solve and find a compromise between energy efficiency and the preservation of cultural heritage. An integrated and careful design process can combine the protection of historical integrity with the ongoing transition to sustainability, without neglecting the indoor comfort for the occupants and the safeguarding of the artworks and frescoes that these buildings often contain. Furthermore, the retrofitting strategies must consider the long-term durability and resilience of the buildings to the climate change.