Building Façade Retroﬁt with Solar Passive Technologies: A Literature Review

: Worldwide, buildings have been presented as one of the main energy consumers and, for that matter, there is an increased tendency to invest in policies and measures that promote more efﬁcient buildings. Among the chosen strategies, the need to promote the use of passive solutions and retroﬁt the existing building stock is often pointed out. Portuguese building stock has proven to be obsolete in terms of thermal comfort, which can directly affect the energy demand for climatization purposes. Considering the great solar availability in the country, when compared to other European locations, building retroﬁt with solar passive technologies can be a suitable solution. This paper aims to review studies on the application of solar passive technologies to retroﬁt façades in the Mediterranean climate context, with a special focus on Portugal. Four retroﬁt passive solar technologies were reviewed, namely glazing, sun shading, sunspaces and Trombe wall technologies.


Introduction
Worldwide, buildings are presented as one of the main energy consumers [1]. Poor design of existing buildings, along with the improvement of living over the years, has contributed to a significant increase in energy consumption for space conditioning [2]. Consequently, several policies and regulations have been designed to promote more efficient buildings. The Energy Performance of Buildings Directive, also known as EPBD, is an example of policies [3][4][5]. It targets not only new but also existing buildings as the European building stock is mostly represented by the existing buildings.
To counter the aging of the existing European building stock, the best solution is to building retrofit [6], which can be seen as an opportunity to reduce energy consumption and improve the thermal comfort and well-being of the occupants [7]. Regarding the Portuguese context, according to the last Census [8], nearly 70% of the residential building stock was built prior to 1990, when the first thermal regulation was set up [9]. As a result, due to insufficient thermal insulation at the opaque building envelope, inappropriate windows (single-glazed windows with timber or aluminum without a thermal break frame) and lack of care with the regard to correction of thermal bridges, most of the buildings are in poor condition regarding thermal performance and energy efficiency. The last Portuguese Census [8] also showed that most of the needs to repair occur at the envelope level (walls, window frames and roof), which can directly impact the buildings' thermal performance. Indeed, buildings built prior to 1990 are responsible for higher energy demands for heating, ranging from 265 to 334 kWh/m 2 .year, while those built between 1990 and 2010 require between 112 and 265 kWh/m 2 .year [10]. According to a study performed in 2014 [11], approximately 80% of residential buildings had energy demands for heating higher than

Solar Availability
The potential of solar passive systems mainly relies on the availability of solar radiation, which is determined by the climate and latitude of the location and by the placement and position of the system [31,32]. Solar availability on urban façades is also impacted by the obstruction caused by nearby buildings [32].
Studies reported in the reviewed literature characterize solar availability either using the global vertical irradiation on the south façade or the global horizontal irradiation [33], [34]. In the Portuguese context, a study based on solar irradiation data devised by Brito et al. [35] analyzed the solar potential of roofs and façades of two different neighbors in Lisbon. According to their study, the annual solar irradiation of roofs varied between 1000 and 1800 kWh/m 2 .year, while façades varied between 100 and 1000 kWh/m 2 .year, depending on the façade orientation. The study also showed that façades facing east and west present a wide range of irradiation values (100-800 kWh/m 2 /year), and the sum of their potential could surpass those facing south.
In this study, the annual Global Solar Irradiation (GSI) is used as an indicator to evaluate solar availability. Figure 1 shows the GSI of Lisbon when compared to other European locations, namely Ancona (Italy), Barcelona (Spain), London (United Kingdom), München (Germany), Paris (France) and Stockholm (Sweden). The data correspond to 2015 and were obtained from the Photovoltaic Geographical Information System of the EU Science Hub of the European Commission [36]. As can be seen in Figure 1, high annual values were recorded in all locations of Mediterranean climates (Lisbon, Ancona and Barcelona), the highest value being associated with Lisbon (1789 kWh/m 2 .year). When compared to Paris, Munich, London and Stockholm, Lisbon's GSI is 31%, 33%, 41% and 48% higher than the corresponding values, respectively.
Energies 2021, 14, x FOR PEER REVIEW 3 of 20 and 1800 kWh/m 2 .year, while façades varied between 100 and 1000 kWh/m 2 .year, depending on the façade orientation. The study also showed that façades facing east and west present a wide range of irradiation values (100-800 kWh/m 2 /year), and the sum of their potential could surpass those facing south. In this study, the annual Global Solar Irradiation (GSI) is used as an indicator to evaluate solar availability. Figure 1 shows the GSI of Lisbon when compared to other European locations, namely Ancona (Italy), Barcelona (Spain), London (United Kingdom), München (Germany), Paris (France) and Stockholm (Sweden). The data correspond to 2015 and were obtained from the Photovoltaic Geographical Information System of the EU Science Hub of the European Commission [36]. As can be seen in Figure 1, high annual values were recorded in all locations of Mediterranean climates (Lisbon, Ancona and Barcelona), the highest value being associated with Lisbon (1789 kWh/m 2 .year). When compared to Paris, Munich, London and Stockholm, Lisbon's GSI is 31%, 33%, 41% and 48% higher than the corresponding values, respectively.

Solar Passive Technologies
In this section, the key findings of the studies devised on the application of solar passive technologies used to retrofit façades of Portuguese buildings, considering the Mediterranean climate, are presented.
As Passive Strategies, solar technologies can address both, heating and cooling needs by preventing, rejecting/collecting, or controlling heat gains. Solar technologies can be categorized as Direct and Indirect Solar Gain Technologies [37]. Direct Gain Solar Technologies absorb, store and release solar energy directly into the indoor environment ( Figure  2a) [38].

Solar Passive Technologies
In this section, the key findings of the studies devised on the application of solar passive technologies used to retrofit façades of Portuguese buildings, considering the Mediterranean climate, are presented.
As Passive Strategies, solar technologies can address both, heating and cooling needs by preventing, rejecting/collecting, or controlling heat gains. Solar technologies can be categorized as Direct and Indirect Solar Gain Technologies [37]. Direct Gain Solar Technologies absorb, store and release solar energy directly into the indoor environment ( Figure 2a) [38]. and 1800 kWh/m 2 .year, while façades varied between 100 and 1000 kWh/m 2 .year, depending on the façade orientation. The study also showed that façades facing east and west present a wide range of irradiation values (100-800 kWh/m 2 /year), and the sum of their potential could surpass those facing south. In this study, the annual Global Solar Irradiation (GSI) is used as an indicator to evaluate solar availability. Figure 1 shows the GSI of Lisbon when compared to other European locations, namely Ancona (Italy), Barcelona (Spain), London (United Kingdom), München (Germany), Paris (France) and Stockholm (Sweden). The data correspond to 2015 and were obtained from the Photovoltaic Geographical Information System of the EU Science Hub of the European Commission [36]. As can be seen in Figure 1, high annual values were recorded in all locations of Mediterranean climates (Lisbon, Ancona and Barcelona), the highest value being associated with Lisbon (1789 kWh/m 2 .year). When compared to Paris, Munich, London and Stockholm, Lisbon's GSI is 31%, 33%, 41% and 48% higher than the corresponding values, respectively.

Solar Passive Technologies
In this section, the key findings of the studies devised on the application of solar passive technologies used to retrofit façades of Portuguese buildings, considering the Mediterranean climate, are presented.
As Passive Strategies, solar technologies can address both, heating and cooling needs by preventing, rejecting/collecting, or controlling heat gains. Solar technologies can be categorized as Direct and Indirect Solar Gain Technologies [37]. Direct Gain Solar Technologies absorb, store and release solar energy directly into the indoor environment ( Figure  2a) [38].   In the case of indirect solar gain technologies, solar energy is first absorbed by a thermal mass and then transferred into the indoor environment for later use (Figure 2b) [37]. A matrix of suitable retrofit solutions for each case is presented in Table 1, and the retrofit solutions are described in detail in Sections 3.1-3.4.  (Table 2).

Glazing Technologies
Worldwide, window retrofit is, without any doubt, the most popular retrofit action. The reasons behind this success are threefold. Firstly, windows usually present higher thermal transmittance values (U-values) than the opaque façade [26], and, as consequence, most of the heat exchange between the exterior and interior environment occurs via glazed spans [39]. Secondly, financial support programs often encourage window retrofit. In the Portuguese context, the "Renew Home & Office" program [40] is an example of such programs. Thirdly, windows are the simplest solar passive technology and the easiest building element to retrofit [20,37].
In the Portuguese context, window retrofit is either done by: • Installing a secondary window in addition to the existing single-glazed window; • Replacing the entire window system; • Adding Solar Control Films (SCFs) to the existing window.
Window retrofit through the installation of a secondary window is based on the internal addition of a second pane of glass or plastic, while keeping the original single-glazed window, to improve the thermal performance of the window system during wintertime [40]. This solution can also maintain the external original aesthetics of the renovated window system, which is useful in the case of retrofitting the historical buildings, as shown in Figure 3, which illustrates the impact of the retrofitting on a building located in the UN-ESCO's Historic Centre of Porto [41]. No measurable impact of this type of retrofit solution was found in the literature for the Portuguese context. In the case of indirect solar gain technologies, solar energy is first absorbed by a thermal mass and then transferred into the indoor environment for later use (Figure 2b) [37]. A matrix of suitable retrofit solutions for each case is presented in Table 1, and the retrofit solutions are described in detail in Sections 3.1-3.4.

Glazing Technologies
Worldwide, window retrofit is, without any doubt, the most popular retrofit action. The reasons behind this success are threefold. Firstly, windows usually present higher thermal transmittance values (U-values) than the opaque façade [26], and, as consequence, most of the heat exchange between the exterior and interior environment occurs via glazed spans [39]. Secondly, financial support programs often encourage window retrofit. In the Portuguese context, the "Renew Home & Office" program [40] is an example of such programs. Thirdly, windows are the simplest solar passive technology and the easiest building element to retrofit [20,37].
In the Portuguese context, window retrofit is either done by: • Installing a secondary window in addition to the existing single-glazed window; • Replacing the entire window system; • Adding Solar Control Films (SCFs) to the existing window.
Window retrofit through the installation of a secondary window is based on the internal addition of a second pane of glass or plastic, while keeping the original singleglazed window, to improve the thermal performance of the window system during wintertime [40]. This solution can also maintain the external original aesthetics of the renovated window system, which is useful in the case of retrofitting the historical buildings, as shown in Figure 3, which illustrates the impact of the retrofitting on a building located in the UNESCO's Historic Centre of Porto [41]. No measurable impact of this type of retrofit solution was found in the literature for the Portuguese context.    Regarding the replacement of window systems, most Portuguese studies considered the replacement of single glazing with multilayer glazings. On the whole, triple-glazed windows were usually chosen for locations in the north and center-north of the mainland, as shown in [42]. However, the replacement with double-glazed windows was the most common solution nationwide for both residential and nonresidential buildings, as shown in [41,[43][44][45][46][47][48][49] and [50,51], respectively. It is estimated that replacing single-with doubleglazed windows is an investment that can cost between 200 and 1500 EUR/unit, whereas the investment in retrofit measures of the opaque walls with traditional External Thermal Insulation Composite Systems (ETICSs) is between 30 and 60 EUR/m 2 . The measurable impact of replacing single-with double-glazed windows was studied by Eskander et al. [47], who verified that the heat transferred through the windows could be lowered to near 40% following the retrofit. Eicker et al. [52] analyzed the impact of replacing glazing systems with ones with lower total solar energy transmittance (g-value), a parameter that is also known as the solar factor. According to [53], the g-value represents the quotient between the energy transmitted to the interior through a glazed gap and the energy of the solar radiation that affects it. The researchers concluded that the cooling energy needs of a residential building in Almada (a city and a municipality in Portugal located on the southern margin of the Tagus River from Lisbon) could be lowered to 11% if single-glazed windows with a g-value of 0.8 were replaced with double-glazed windows with a g-value of 0.51 [52].
Other studies considered the replacement of single-glazed windows with doubleglazed and low-emissive (e-low) glazing windows [42,54]. The literature pointed out that e-low glazing is a good option for a mild-temperate climate (Portugal mainland) because it provides good thermal insulation during wintertime and reduces the entrance of heat during summertime, allowing the passage of visible light for daylight [38,55].
As stated by Moretti and Belloni [56], to encourage visual comfort and daylighting for energy savings, nonresidential buildings tend to have high window-to-wall ratios. However, this design option may also lead to an increase in energy demand for heating or cooling purposes. For that reason, in recent years, many studies have been devised on glazing retrofit with Solar Control Films (SCFs) [57][58][59]. In Portugal, Teixeira et al. [59] studied an office building with large glazed areas in Lisbon and concluded that SCFs can be a suitable retrofit measure as they can reduce solar heat gains, the peak demand load and the annual energy consumption. These findings are pertinent because, as Causone et al. [60] pointed out, large glazed surfaces can cause a significant increase of cooling loads in warm climates due to the increased solar heat gain. Furthermore, Pereira et al. [57] also studied the application of SCF in the Lisbon context and verified that this retrofit solution has the potential to promote daylight availability while limiting the visual discomfort in office buildings ( Figure 4).
Regarding the replacement of window systems, most Portuguese studies considered the replacement of single glazing with multilayer glazings. On the whole, triple-glazed windows were usually chosen for locations in the north and center-north of the mainland, as shown in [42]. However, the replacement with double-glazed windows was the most common solution nationwide for both residential and nonresidential buildings, as shown in [41,[43][44][45][46][47][48][49] and [50,51], respectively. It is estimated that replacing single-with doubleglazed windows is an investment that can cost between 200 and 1500 EUR/unit, whereas the investment in retrofit measures of the opaque walls with traditional External Thermal Insulation Composite Systems (ETICSs) is between 30 and 60 EUR/m 2 . The measurable impact of replacing single-with double-glazed windows was studied by Eskander et al. [47], who verified that the heat transferred through the windows could be lowered to near 40% following the retrofit. Eicker et al. [52] analyzed the impact of replacing glazing systems with ones with lower total solar energy transmittance (g-value), a parameter that is also known as the solar factor. According to [53], the g-value represents the quotient between the energy transmitted to the interior through a glazed gap and the energy of the solar radiation that affects it. The researchers concluded that the cooling energy needs of a residential building in Almada (a city and a municipality in Portugal located on the southern margin of the Tagus River from Lisbon) could be lowered to 11% if single-glazed windows with a g-value of 0.8 were replaced with double-glazed windows with a g-value of 0.51 [52].
Other studies considered the replacement of single-glazed windows with doubleglazed and low-emissive (e-low) glazing windows [42,54]. The literature pointed out that e-low glazing is a good option for a mild-temperate climate (Portugal mainland) because it provides good thermal insulation during wintertime and reduces the entrance of heat during summertime, allowing the passage of visible light for daylight [38,55].
As stated by Moretti and Belloni [56], to encourage visual comfort and daylighting for energy savings, nonresidential buildings tend to have high window-to-wall ratios. However, this design option may also lead to an increase in energy demand for heating or cooling purposes. For that reason, in recent years, many studies have been devised on glazing retrofit with Solar Control Films (SCFs) [57][58][59]. In Portugal, Teixeira et al. [59] studied an office building with large glazed areas in Lisbon and concluded that SCFs can be a suitable retrofit measure as they can reduce solar heat gains, the peak demand load and the annual energy consumption. These findings are pertinent because, as Causone et al. [60] pointed out, large glazed surfaces can cause a significant increase of cooling loads in warm climates due to the increased solar heat gain. Furthermore, Pereira et al. [57] also studied the application of SCF in the Lisbon context and verified that this retrofit solution has the potential to promote daylight availability while limiting the visual discomfort in office buildings ( Figure 4).

Sun Shading Technologies
Portuguese cities such as Évora and Beja are characterized by very hot summers, with average temperatures of 30.2 C and 32.6 • C, respectively, while Lisbon has an average maximum temperature of 27.4 • C in July [61,62]. Thermal discomfort due to overheating, during the cooling season, is often pointed as a key issue, as shown in [63]. For this reason, the control of solar heating gains is a key retrofit measure in the Portuguese context [64]. The most effective way to control solar heat gain is by intercepting it before it reaches the building envelope while ensuring that the strategy does not interfere with solar gains during wintertime when it is required [65]. This control can be achieved using sun shading devices, as shown in [66,67]. This solar technology is often considered and recommended in building retrofit studies, as shown in [42,68,69].
Capeluto et al. [70] analyzed the retrofit of façades in multiple European cities and concluded that, when a single strategy was chosen to retrofit a south-facing façade in Porto, sun shading was the best option. The study also showed that, even when multiple strategies are chosen, sun shading appears twice in the top three combined measures, first combined with glazing and in third combined with ventilation.
Studies on the individual impact of sun shading retrofitting in the Portuguese context are scarce. It appears from the literature that, with very few exceptions, this retrofit option was always considered in conjunction with other retrofit measures. In other words, the specific measurable impact of sun shading retrofit was rarely presented and discussed. One of the exceptions was presented by Pereira Tavares et al. [38], who assessed the impact of adding horizontal brise-soleils of different sizes to housing units in Lisbon ( Figure 5). The authors concluded that by adding horizontal brise-soleils to buildings with large glazing areas, the cooling energy needs could be lowered by up to 55%, depending on the façade orientation and the type of glass of the window. The study also showed that the greater the width of the used shading devices, the lower the influence of the type of glass of the window.

Sun Shading Technologies
Portuguese cities such as Évora and Beja are characterized by very hot summers, with average temperatures of 30.2 C and 32.6 °C, respectively, while Lisbon has an average maximum temperature of 27.4 °C in July [61,62]. Thermal discomfort due to overheating, during the cooling season, is often pointed as a key issue, as shown in [63]. For this reason, the control of solar heating gains is a key retrofit measure in the Portuguese context [64]. The most effective way to control solar heat gain is by intercepting it before it reaches the building envelope while ensuring that the strategy does not interfere with solar gains during wintertime when it is required [65]. This control can be achieved using sun shading devices, as shown in [66,67]. This solar technology is often considered and recommended in building retrofit studies, as shown in [42,68,69].
Capeluto et al. [70] analyzed the retrofit of façades in multiple European cities and concluded that, when a single strategy was chosen to retrofit a south-facing façade in Porto, sun shading was the best option. The study also showed that, even when multiple strategies are chosen, sun shading appears twice in the top three combined measures, first combined with glazing and in third combined with ventilation.
Studies on the individual impact of sun shading retrofitting in the Portuguese context are scarce. It appears from the literature that, with very few exceptions, this retrofit option was always considered in conjunction with other retrofit measures. In other words, the specific measurable impact of sun shading retrofit was rarely presented and discussed. One of the exceptions was presented by Pereira Tavares et al. [38], who assessed the impact of adding horizontal brise-soleils of different sizes to housing units in Lisbon ( Figure 5). The authors concluded that by adding horizontal brise-soleils to buildings with large glazing areas, the cooling energy needs could be lowered by up to 55%, depending on the façade orientation and the type of glass of the window. The study also showed that the greater the width of the used shading devices, the lower the influence of the type of glass of the window. Another exception was presented by Palmero-Marrero et al. [71], who analyzed the effect of louver shading devices on building energy requirements of multiple cities across the world, including Lisbon. The authors showed that 50% of the total energy consumption could be saved by using louvers (Figure 6), compared with no shading case. Nevertheless, both studies pointed that adding shading devices can lead to higher energy expenditure during the heating season.  Another exception was presented by Palmero-Marrero et al. [71], who analyzed the effect of louver shading devices on building energy requirements of multiple cities across the world, including Lisbon. The authors showed that 50% of the total energy consumption could be saved by using louvers (Figure 6), compared with no shading case. Nevertheless, both studies pointed that adding shading devices can lead to higher energy expenditure during the heating season.

Sun Shading Technologies
Portuguese cities such as Évora and Beja are characterized by very hot summers, with average temperatures of 30.2 C and 32.6 °C, respectively, while Lisbon has an average maximum temperature of 27.4 °C in July [61,62]. Thermal discomfort due to overheating, during the cooling season, is often pointed as a key issue, as shown in [63]. For this reason, the control of solar heating gains is a key retrofit measure in the Portuguese context [64]. The most effective way to control solar heat gain is by intercepting it before it reaches the building envelope while ensuring that the strategy does not interfere with solar gains during wintertime when it is required [65]. This control can be achieved using sun shading devices, as shown in [66,67]. This solar technology is often considered and recommended in building retrofit studies, as shown in [42,68,69].
Capeluto et al. [70] analyzed the retrofit of façades in multiple European cities and concluded that, when a single strategy was chosen to retrofit a south-facing façade in Porto, sun shading was the best option. The study also showed that, even when multiple strategies are chosen, sun shading appears twice in the top three combined measures, first combined with glazing and in third combined with ventilation.
Studies on the individual impact of sun shading retrofitting in the Portuguese context are scarce. It appears from the literature that, with very few exceptions, this retrofit option was always considered in conjunction with other retrofit measures. In other words, the specific measurable impact of sun shading retrofit was rarely presented and discussed. One of the exceptions was presented by Pereira Tavares et al. [38], who assessed the impact of adding horizontal brise-soleils of different sizes to housing units in Lisbon ( Figure 5). The authors concluded that by adding horizontal brise-soleils to buildings with large glazing areas, the cooling energy needs could be lowered by up to 55%, depending on the façade orientation and the type of glass of the window. The study also showed that the greater the width of the used shading devices, the lower the influence of the type of glass of the window. Another exception was presented by Palmero-Marrero et al. [71], who analyzed the effect of louver shading devices on building energy requirements of multiple cities across the world, including Lisbon. The authors showed that 50% of the total energy consumption could be saved by using louvers (Figure 6), compared with no shading case. Nevertheless, both studies pointed that adding shading devices can lead to higher energy expenditure during the heating season.  Regarding the design of sun shading, the literature emphasized several aspects. The adaptability should be weighed, namely if the shading device is fixed or movable. The aesthetics and handling of shading devices should also be taken into account because they are often pointed as key factors for users [72,73]. As shading devices can reduce natural light, designers should assess the balance between the energy savings from cooling (due to shading devices) and the energy consumption due to artificial light.

Sunspace Technologies
Sunspace is a solar technology designed to collect heat gains. It is based on a glasshouse (Figure 7) that absorbs solar radiation and partially converts it into energy for heating the indoor environment [74][75][76][77].
Energies 2021, 14, x FOR PEER REVIEW 7 of Regarding the design of sun shading, the literature emphasized several aspects. Th adaptability should be weighed, namely if the shading device is fixed or movable. Th aesthetics and handling of shading devices should also be taken into account because the are often pointed as key factors for users [72,73]. As shading devices can reduce natur light, designers should assess the balance between the energy savings from cooling (du to shading devices) and the energy consumption due to artificial light.

Sunspace Technologies
Sunspace is a solar technology designed to collect heat gains. It is based on a glas house (Figure 7) that absorbs solar radiation and partially converts it into energy for hea ing the indoor environment [74][75][76][77]. It recreates on a small scale the macroscopic phenomenon of the greenhouse effe that happens between the Earth and the atmosphere, and it works as a buffer zone again heat losses toward the outdoor environment [74,75]. Portuguese sunspaces are usual south oriented, with single glazing and a high glazing to floor surface ratio (between 60 and 250%) [78].
Case studies on Portuguese retrofitted sunspaces are scarce, even though it is com mon to close existing balconies so that they can perform as sunspaces. Such an examp was studied by Macieira et al. [79], who proposed the use of transparent membran screens to close existing balconies (Figure 8a). To optimize the thermal performance, th proposed retrofit solution can be completely open all day or night time, and it can als integrate an additional opaque/reflective membrane to work as a vertical shade (with a justable height), as shown in Figure 8b. As the proposed retrofit solution uses membran of PVC or EFTE as an alternative to conventional glazing, the technology was name Membrane Alternative Sunspace (MAS). The study concluded that MAS is an efficie alternative to traditional sunspaces, according to constructive, environmental and ec nomic aspects. It recreates on a small scale the macroscopic phenomenon of the greenhouse effect that happens between the Earth and the atmosphere, and it works as a buffer zone against heat losses toward the outdoor environment [74,75]. Portuguese sunspaces are usually south oriented, with single glazing and a high glazing to floor surface ratio (between 60% and 250%) [78].
Case studies on Portuguese retrofitted sunspaces are scarce, even though it is common to close existing balconies so that they can perform as sunspaces. Such an example was studied by Macieira et al. [79], who proposed the use of transparent membrane screens to close existing balconies (Figure 8a). To optimize the thermal performance, the proposed retrofit solution can be completely open all day or night time, and it can also integrate an additional opaque/reflective membrane to work as a vertical shade (with adjustable height), as shown in Figure 8b. As the proposed retrofit solution uses membranes of PVC or EFTE as an alternative to conventional glazing, the technology was named Membrane Alternative Sunspace (MAS). The study concluded that MAS is an efficient alternative to traditional sunspaces, according to constructive, environmental and economic aspects. Aelenei et al. [80] studied the use of different attached-sunspace configurations (Figure 9 in retrofitting design and concluded that fully integrated sunspaces led to better-performing results because they have a smaller glazing area in contact with the external environment. The study also pointed that sunspaces may be a very efficient solution in the south of mainland Portugal (e.g., Faro region), as it evidenced a potential of 100% energy saving during wintertime. However, in locations with more severe winters, such as Bragança, the potential energy savings is lowered to 48%. Furthermore, sunspaces might be a very attractive solution to retrofit dwellings built before the 1990s when compared to retrofit approaches based on thermal insulation due to the good solar potential in Portugal and the economic benefits of sunspace systems [80].

Trombe Wall Technologies
Trombe wall, also known as a storage wall or a solar heating wall, is a solar technology mainly designed to collect heat gains. It consists of an external transparent layer in front of the external wall (known as a massive wall), with a small air cavity between them. The system works by absorbing solar energy through the external transparent skin, which results in a buffer effect in the air cavity, where the heat is stored during the daytime [81][82][83].In the hours following the sunset, the energy is transmitted as heat to the indoor environment, which enhances the indoor air temperature [81][82][83].
According to the ventilation mode of the air cavity of the Trombe wall system, it can be classified as nonventilated (or unvented), ventilated or double ventilated. The ventilation mode relies on the existence of vents (ventilation openings) on the external transparent skin or the massive wall. When the Trombe wall is designed without vents, or the vents are closed, the system is classified as unvented. If the Trombe wall has (open) vents in one layer of the system (e.g., external transparent skin or massive wall), it is classified as ventilated. When the Trombe wall has (open) vents in different two layers of the system, namely on the external transparent skin and on the massive wall, the system is classified as double ventilated. Each one of the described ventilation modes of the Trombe Aelenei et al. [80] studied the use of different attached-sunspace configurations (Figure 9) in retrofitting design and concluded that fully integrated sunspaces led to betterperforming results because they have a smaller glazing area in contact with the external environment. The study also pointed that sunspaces may be a very efficient solution in the south of mainland Portugal (e.g., Faro region), as it evidenced a potential of 100% energy saving during wintertime. However, in locations with more severe winters, such as Bragança, the potential energy savings is lowered to 48%. Furthermore, sunspaces might be a very attractive solution to retrofit dwellings built before the 1990s when compared to retrofit approaches based on thermal insulation due to the good solar potential in Portugal and the economic benefits of sunspace systems [80]. Aelenei et al. [80] studied the use of different attached-sunspace configurations (Figure 9 in retrofitting design and concluded that fully integrated sunspaces led to better-performing results because they have a smaller glazing area in contact with the external environment. The study also pointed that sunspaces may be a very efficient solution in the south of mainland Portugal (e.g., Faro region), as it evidenced a potential of 100% energy saving during wintertime. However, in locations with more severe winters, such as Bragança, the potential energy savings is lowered to 48%. Furthermore, sunspaces might be a very attractive solution to retrofit dwellings built before the 1990s when compared to retrofit approaches based on thermal insulation due to the good solar potential in Portugal and the economic benefits of sunspace systems [80].

Trombe Wall Technologies
Trombe wall, also known as a storage wall or a solar heating wall, is a solar technology mainly designed to collect heat gains. It consists of an external transparent layer in front of the external wall (known as a massive wall), with a small air cavity between them. The system works by absorbing solar energy through the external transparent skin, which results in a buffer effect in the air cavity, where the heat is stored during the daytime [81][82][83].In the hours following the sunset, the energy is transmitted as heat to the indoor environment, which enhances the indoor air temperature [81][82][83].
According to the ventilation mode of the air cavity of the Trombe wall system, it can be classified as nonventilated (or unvented), ventilated or double ventilated. The ventilation mode relies on the existence of vents (ventilation openings) on the external transparent skin or the massive wall. When the Trombe wall is designed without vents, or the vents are closed, the system is classified as unvented. If the Trombe wall has (open) vents in one layer of the system (e.g., external transparent skin or massive wall), it is classified as ventilated. When the Trombe wall has (open) vents in different two layers of the system, namely on the external transparent skin and on the massive wall, the system is classified as double ventilated. Each one of the described ventilation modes of the Trombe wall is illustrated in Figure 10.

Trombe Wall Technologies
Trombe wall, also known as a storage wall or a solar heating wall, is a solar technology mainly designed to collect heat gains. It consists of an external transparent layer in front of the external wall (known as a massive wall), with a small air cavity between them. The system works by absorbing solar energy through the external transparent skin, which results in a buffer effect in the air cavity, where the heat is stored during the daytime [81][82][83]. In the hours following the sunset, the energy is transmitted as heat to the indoor environment, which enhances the indoor air temperature [81][82][83].
According to the ventilation mode of the air cavity of the Trombe wall system, it can be classified as nonventilated (or unvented), ventilated or double ventilated. The ventilation mode relies on the existence of vents (ventilation openings) on the external transparent skin or the massive wall. When the Trombe wall is designed without vents, or the vents are closed, the system is classified as unvented. If the Trombe wall has (open) vents in one layer of the system (e.g., external transparent skin or massive wall), it is classified as ventilated. When the Trombe wall has (open) vents in different two layers of the system, namely on the external transparent skin and on the massive wall, the system is classified as double ventilated. Each one of the described ventilation modes of the Trombe wall is illustrated in Figure 10. Trombe wall technologies are not commonly used to retrofit residential buildings in Portugal, perhaps due to the lack of information among Portuguese building designers on the application and efficiency of this constructive solution [85]. Nevertheless, among solar technologies, a Trombe wall seems to be a suitable retrofit solution for residential buildings due to its simple configuration, zero running costs, among other reasons [86,87].
As Trombe wall systems are not commonly used in Portugal, studies on their application and impact are also scarce. The studies developed by Briga-Sá et al. [84,85,88] are one of the few exceptions. With regard to the performance, the researchers concluded that the integration of a Trombe wall is beneficial for the performance of Portuguese buildings, given the fact that the energy needs for heating can be reduced up to approximately 16% [85]. Other key findings have also been presented in the literature with regard to the application of Trombe wall systems in European locations with Mediterranean climates. For example, the optimized design of a Trombe wall is often a double-glazed layer, as seen in [33,84] and [88][89][90][91], with air layer depths lower than 10 cm, as seen in [88,89,92]. In addition, during summertime is strongly recommended the use of sun shading to counter overheating. This statement was referred to both in Portuguese studies [84,90] and studies devised on other European regions with Mediterranean climates [89,93]. For example, Briga-Sá et al. [84] noticed that the outer surface of the Trombe wall can reach temperatures above 60 °C when it is unscreened but under 30 °C when it is shaded.
In recent years, the literature also presented some innovative Trombe wall technologies considering the Portuguese context. Sacht et al. [90] investigated the performance of a modular Trombe wall ( Figure 11) for building retrofit under different Portuguese climates. The proposed retrofit solution is innovative for two reasons. Firstly, it is based on multiple modules of a Trombe wall, instead of a single zone, and modules are normally used in active system devices (e.g., PV modules). In addition, the study showed that, regardless of the location, when the modular Trombe wall was considered, the heating energy demand met the Portuguese building code requirements. Trombe wall technologies are not commonly used to retrofit residential buildings in Portugal, perhaps due to the lack of information among Portuguese building designers on the application and efficiency of this constructive solution [85]. Nevertheless, among solar technologies, a Trombe wall seems to be a suitable retrofit solution for residential buildings due to its simple configuration, zero running costs, among other reasons [86,87].
As Trombe wall systems are not commonly used in Portugal, studies on their application and impact are also scarce. The studies developed by Briga-Sá et al. [84,85,88] are one of the few exceptions. With regard to the performance, the researchers concluded that the integration of a Trombe wall is beneficial for the performance of Portuguese buildings, given the fact that the energy needs for heating can be reduced up to approximately 16% [85]. Other key findings have also been presented in the literature with regard to the application of Trombe wall systems in European locations with Mediterranean climates. For example, the optimized design of a Trombe wall is often a double-glazed layer, as seen in [33,84,[88][89][90][91], with air layer depths lower than 10 cm, as seen in [88,89,92]. In addition, during summertime is strongly recommended the use of sun shading to counter overheating. This statement was referred to both in Portuguese studies [84,90] and studies devised on other European regions with Mediterranean climates [89,93]. For example, Briga-Sá et al. [84] noticed that the outer surface of the Trombe wall can reach temperatures above 60 • C when it is unscreened but under 30 • C when it is shaded.
In recent years, the literature also presented some innovative Trombe wall technologies considering the Portuguese context. Sacht et al. [90] investigated the performance of a modular Trombe wall ( Figure 11) for building retrofit under different Portuguese climates. The proposed retrofit solution is innovative for two reasons. Firstly, it is based on multiple modules of a Trombe wall, instead of a single zone, and modules are normally used in active system devices (e.g., PV modules). In addition, the study showed that, regardless of the location, when the modular Trombe wall was considered, the heating energy demand met the Portuguese building code requirements.
Another innovative technology within Trombe wall systems was presented in the literature by Aelenei et al. [94]. They proposed a complementary solution to a traditional External Thermal Insulation Composite System (ETICS), which targets thermal bridge areas with the intent to mitigate winter heat losses ( Figure 12). The proposed solution relies on the principle of unvented Trombe walls, but it considers the external transparent skin in front of a structural thermal bridge area (boundary between concrete columns and masonry walls). The design of this innovative concept, named Solar Bridge Retrofit System (SBRS), enables heavy structures of the building (e.g., concrete columns) to absorb solar radiation and conduct the heat slowly inward or to the adjacent structure [94]. Another innovative technology within Trombe wall systems was presented in the literature by Aelenei et al. [94]. They proposed a complementary solution to a traditional External Thermal Insulation Composite System (ETICS), which targets thermal bridge areas with the intent to mitigate winter heat losses ( Figure 12). The proposed solution relies on the principle of unvented Trombe walls, but it considers the external transparent skin in front of a structural thermal bridge area (boundary between concrete columns and masonry walls). The design of this innovative concept, named Solar Bridge Retrofit System (SBRS), enables heavy structures of the building (e.g., concrete columns) to absorb solar radiation and conduct the heat slowly inward or to the adjacent structure [94].    Another innovative technology within Trombe wall systems was presented in the literature by Aelenei et al. [94]. They proposed a complementary solution to a traditional External Thermal Insulation Composite System (ETICS), which targets thermal bridge areas with the intent to mitigate winter heat losses ( Figure 12). The proposed solution relies on the principle of unvented Trombe walls, but it considers the external transparent skin in front of a structural thermal bridge area (boundary between concrete columns and masonry walls). The design of this innovative concept, named Solar Bridge Retrofit System (SBRS), enables heavy structures of the building (e.g., concrete columns) to absorb solar radiation and conduct the heat slowly inward or to the adjacent structure [94].

Feasibility of Retrofit Works with Solar Passive Technologies
Each building is unique and, thus, it is the nature of its retrofit project, which makes it difficult to define general action guidelines and implies that the optimal solution should be defined for each retrofit case [6]. To facilitate the process of finding the optimal retrofit solutions for each case, the literature presented assessment methodologies on the feasibility of the retrofit works. Those methodologies, such as those presented in [95,96], were designed to assess any kind of retrofit works and not specifically the retrofit works with solar passive technologies. Nevertheless, the performance indicators associated with those methodologies may be used to assess the feasibility of the solar passive technologies listed in Section 3 as well. Table 3 presents a list of indicators that can help assess the feasibility of retrofit projects with solar passive technologies. It should be noted that retrofit measures usually lead to lower energy needs, but that does not always imply a lower global cost [95]. Therefore, studies on the feasibility should not only be based on energy efficiency indicators but also thermal performance and comfort, indoor air quality and acoustics, fire protection and environment and economic indicators. Furthermore, retrofit design projects should also include consideration of the following aspects:

•
Retrofit conditions-questions such as the disturbance levels to the inhabitants or site and the possibility of working from the inside of the building vs. the need for scaffoldings in high-rise buildings should be holistically weighed, as shown in [97]. • Compatibility between the existing constructive solutions and the proposing retrofit technologies-the retrofit design project should be compatible with the existing solution (e.g., materials properties), and possible consequences should be assessed (e.g., reduction of air permeability and the creation of new thermal bridge areas following the fixation of new constructive systems), as well as how to mitigate them [98]; • Impact on cultural heritage-the retrofit design project should involve a strong dichotomy between aesthetic-architectonical value and energy efficiency goals, especially in the context of the rehabilitation of historic buildings [99].

Conclusions
This paper presents an overview of the state-of-art solar passive technologies used to retrofit Portuguese façades. The final remarks and recommendations for future works are presented as follows: 1.
Direct solar technologies (glazing and sun shading) are commonly used strategies to thermally retrofit Portuguese façades, and they are often applied together. Their widespread application is assisted by: (a) their simplicity (easy elements to retrofit); (b) the Portuguese regulation that presents design parameters (which facilitates the job of building designers); (c) national financial support programs (incentives). Sun shading is often recommended in the literature, but its specific impact is usually not indicated.

2.
There is an apparent scarcity of studies on indirect solar technologies used to retrofit Portuguese buildings. Regarding sunspaces, even though balconies are often converted into sunspaces, there are no sufficient available data on the optimization design parameters leading to the upgrade of the energy performance. Regarding Trombe wall systems, most of the studies found were devised in the last decade, meaning that there is a growing interest in this subject. All in all, more detailed studies and research (on optimization design, operation, and efficiency) should be devised on Trombe wall systems to encourage building designers and owners to consider this technology.

3.
Studies have shown that solar passive technologies usually lead to energy savings. However, more detailed feasibility studies should be conducted, especially concerning cost and environmental indicators. Specific studies on the feasibility of solar passive technologies compared to conventional retrofit solutions should also be conducted to further increase the knowledge on solar passive technologies in the Portuguese context.

4.
Overall, it was concluded that building retrofit with solar passive technologies may be a viable way to (a) improve the thermal comfort and efficiency (more energy savings and lower greenhouse gas emissions) of buildings in the Mediterranean climate context, (b) further improve the technical and scientific knowledge, (c) increase the building value as it updates it toward current living standards and (d) contribute toward achieving greener buildings and cities. Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.