Critical review of current understanding of passive façade design in residential buildings

The focus domain of the first cluster is on the aesthetic quality of architectural design which has been studied by several researchers (Akalin et al 2010, Kozlova 2018, Azemati et al 2020, Ghomeishi 2021, Jam et al 2021). Several parameters were found to have significant influence on aesthetic perception and individual preferences. Ghomeishi (2021) identified originality meaningfulness, clarity, and simplicity while Akalin et al (2010) cited preference, complexity and impressiveness, and Azemati et al (2020) reported symmetry and expertise as being important factors influencing perception. However, there is a significant difference in aesthetic judgement among experts (architects) and non-experts (laymen) (Azemati et al 2020, Ghomeishi 2021, Jam et al 2021). Experts preferred simple and asymmetrical designs while non-experts preferred symmetric and more complex building façades. For both groups, experts and non-experts intermediate level of complexity was the most preferred design (Akalin et al 2010, Azemati et al 2020, Jam et al 2021). Furthermore, Kozlova (2018) emphasized the concept of videoecology, where the visual quality of the architectural environment is influenced by both, ecology (natural urban environment) and aesthetic quality. Monotony (Azemati et al 2020) and heterogeneity (Kozlova 2018), manifested as meaningless repetitions typical of modern cities, should be avoided since they may cause psychological and visual fatigue.

Another research area is acoustics where the importance of façade design on acoustic comfort is investigated (Jurevicius et al 2016, Meza et al 2019, Alonso et al 2021, Eggenschwiler et al 2022). The studies mainly evaluated the efficacy of façade treatments to mitigate traffic noise. Meza et al (2019) found that window types, window to wall ratio, condition of seals, presence of cracks, and the overall quality of the façade construction affect acoustic performance, while Eggenschwiler et al (2022) proved that absorbing façade surface and architectural morphology could improve outdoor and indoor acoustic quality. The studies also pointed out that there is a lack of local (Meza et al 2019) and global regulations (Alonso et al 2021).

While structure is a broad topic, the selected literature investigates seismic resistance of building façades. The studies found that in earthquake prone areas reinforced concrete buildings could be strengthened by adding shear walls and other structural members which do not alter the geometric proportions of the façade (Toker and Ünay 2006). Tusnina and Emelianov (2018) found that hinged composite cladding panels demonstrated increased seismic stability due to their lightweight and reduction of seismic loads. In addition to seismic resistance, passive fire resistance was also studied. It was found that more expensive façade structures performed better in delaying the spread of fire and provide better seismic and thermal protection (Jakšić et al 2023).

Another common research area is the development of a sustainability framework, namely LCA to conduct an environmental assessment of different façade designs on existing buildings including façade systems (Ansah et al 2020), façade materials (Petrovski et al 2020) and other façade parameters (Radhi and Sharples 2013). In doing so, these studies have investigated the reduction of embodied energy and operational energy and provided some guidelines in designing façade systems that reduce environmental impacts. However, due to an ageing building stock, particularly in Europe, there are numerous studies that focus on developing a LCA for building façade retrofit and refurbishment. To improve energy efficiency of European housing stock, retrofitting options with thermal insulation were studied (Sattler and Österreicher 2019, Bitar et al 2022). Construction and demolition waste, as well as design for disassembly is also considered an important life cycle phase of a refurbished building (Zhang et al 2020). Meanwhile, Blom et al (2010) discuss the role of maintenance, service life and durability in the LCA process. As part of a comprehensive LCA, some researchers have included a LCC. Ansah et al (2020) reported a 39% reduction in energy demand, and a reduction of 47% in LCC. Economic viability of different façade designs is evaluated for new built scenarios (Ansah et al 2020), retrofitting existing buildings (Zhang et al 2020), and the integration of solar panels (Italos et al 2022). Evaluated parameters include investment costs, maintenance and service costs, operation cost, replacement costs, and demolition costs, as well as a calculated payback period (Italos et al 2022).

The diagnosis of building defects represents another research cluster. Chew (2021) demonstrated a systematic façade inspection to evaluate the risk of falling objects from tall buildings posing a public safety hazard. Likewise, Diaz et al (2020) used manual inspection to define the most common risk parameters that cause defects. Their findings suggest that current building regulations are inadequate at preventing common defects and urge future building regulations to set a maximum distance between movement joints. However, there are academics that have criticised traditional defect detection by an inspector, citing the process as labour intensive, time consuming, subjective and highly inaccurate (Lee et al 2021, 2022). To address this issue, Lee et al (2020) proposed a façade monitoring system which uses object detection method that is based on deep learning to accurately detect defects while conserving manpower. An accuracy level of 63% is reached by using a convolutional neural network (Faster R-CNN) (Lee et al 2020). Lee et al (2022) further proposed a bounding-box object augmentation method that enhances the accuracy of deep learning models in the detection of façade defects.

The role of natural ventilation in improving thermal comfort is another topic of interest among researchers. They (Wang and Wong 2006, Wang et al 2007, Ananacha et al 2013, Tong et al 2019) found that for study locations situated in the tropical climate, such as Singapore and Thailand, enhancing natural ventilation is most effective in improving indoor thermal comfort. Wang et al (2007) reported that North and South facing windows with a window to wall ratio of 0.24 and horizontal shading significantly improved indoor thermal comfort. Tong et al (2019) verified through experimental study that the temperature difference between closed and opened window can reach around 8 °C, Ananacha et al (2013) demonstrated that a façade design optimised for natural ventilation could reduce indoor temperatures by up to 3 °C. In high-rise apartments, natural ventilation can cause discomfort and safety issues since outdoor windspeed, and pressure intensifies with building height (Leigh et al 2004, Cho et al 2013). To address this problem, Cho et al (2013) proposed ventilated DSF as an alternative. An optimised DSF can increase natural ventilation rates by 1.5 times (Cho et al 2013). Cooling loads can be lowered by 30%–40% (Leigh et al 2004), and around 15% (Xu and Ojima 2007), respectively by maximizing natural ventilation through ventilated DSF.

The most researched cluster is building performance and energy efficiency. It is, therefore, a broad area of expertise, and researchers studied different areas of performance and energy conservation ranging from energy retrofitting, use of passive design strategies, materials, and specific façade systems such as DSFs, green façades etc. Concerning energy retrofit, the most common strategies identified are adding thermal insulation (Curado and de Freitas 2019, Calama-González et al 2022), adding a new energy efficient façade system onto the existing façade (Fotopoulou et al 2018, Paiho et al 2019), developing new façade component details (Gali Taşçi et al 2017), using prefabricated panels (Sousa 2013) and optimizing shading and aperture design (dos Santos Dolce Uzum and Soares Gonçalves 2021). The prefabricated multifunctional energy efficient façade system was found to be effective in colder climates, reducing heating loads by around 65% compared to a non-retrofitted façade (Paiho et al 2019). Gali Taşçi et al (2017) tested several retrofit strategies ranging from glazing replacement, insulation thickness, wall thickness and shading. It was reported that the optimized façade component design which combines these strategies could reduce cooling loads by approximately 16%, and heating loads by 42%, respectively in a mediterranean climate (Gali Taşçi et al 2017). While adding thermal insulation is beneficial for energy retrofitting in a cold climate, the results are inconclusive for warmer climates. In countries such as Spain and Portugal with a mediterranean climate, the application of the external thermal insulation composite system (ETICS) showed annual reductions in total energy load and discomfort hours (Calama-González et al 2022). However, during the warm seasons there is no substantial improvement in indoor conditions by adding thermal insulation, and for some cities there could be an increase in discomfort hours (Curado and de Freitas 2019). Since heatwaves will become more frequent and severe due to climate change, the ETICS systems need to be carefully studied and optimized (Calama-González et al 2022). In the tropical climate of Sao Paolo, it was reported that external shading and aperture design were the most energy efficient retrofitting strategies with decrease in discomfort hours from 25% to 7% (dos Santos Dolce Uzum and Soares Gonçalves 2021).

Meanwhile, Liu et al (2020) found passive design strategies could effectively reduce energy loads in Hong Kong which has a hot and humid tropical climate. By employing passive façade and envelope design strategies could read the annual and cooling loads by up to 57% and 65%, respectively. In light of the different climate change scenarios, Liu et al (2020) further suggested that building airtightness will more than triple in importance, and that the cooling potential of natural ventilation will decrease when the outdoor environment becomes hotter.

The energy saving potential of a DFS is a topic of interest among researchers. Substantial amount of literature suggests that DSF in cold and temperate climates is an effective solution. Yoon et al (2023) reported annual savings of up to 39% in heating loads for a building located in a cold climate. However, several studies also pointed at the efficacy of DSF in a warmer climate. Leigh et al (2004) reported cooling load savings of 30%–40%, while Hay and Ostertag (2018) stated a 9.2% reduction in annual operational CO₂ emissions. Likewise, Calama-González et al (2022) reported that DSF performs better in Summer when outdoor temperatures are above 35 °C during daytime, but has poor heat dissipation during night-time.

VGS have been studied for their cooling potential in buildings and the urban microclimatic environment. In the temperate climate of Netherlands, Nguyen et al (2019) reported the application of a green façade led to a 8 °C reduction in surface temperature, 1 °C decrease in indoor temperature and 35% cooling load reduction. Alsaad et al (2022) reported similar findings where a green façade applied in Germany could lower surface temperatures by 7 °C and reduce indoor air temperatures by over 3 °C. In hotter climate conditions, similar benefits were observed. Kumar et al (2019) cited a heat mitigation potential of 40% when using a green façade with coir mat. Similarly, El-Zoklah and Refaat (2021) observed annual energy savings of 24%, heating load reduction by 22% and cooling reduction by 24%, respectively.

In terms of data related research gaps, these were mostly reported in the domain of deep learning. Researchers studied the use of deep learning in façade defect detection. It was found that the dataset is too limited to develop an automated deep learning model for defect detection (Lee et al 2021). Likewise, Jones et al (2014) reported a need to expand the database for training and increase data processing abilities to improve the accuracy of the defect detection model proposed in his study. For another type of deep learning model that generate façade design for energy conservation, similar data related problems were observed. Researchers cited database limitations, lack of data accuracy and data processing as requiring further consideration (Martinez and Choi 2017, Wan et al 2023). It was also observed that there were data related limitations in LCA. Data-related gaps were reported in the lack of availability of data for certain materials, and a lack of data their installation and disposal, which led to uncertainties, and inaccuracies of the LCA study outcomes (Hay and Ostertag 2018, Zhang et al 2020).

In addition to data related gaps, methodological research gaps were observed in numerous studies. In the domain of energy efficiency and thermal performance, it was reported that numerical simulations (Sánchez et al 2019) and building energy simulations lacked accuracy and as such a verification was necessary through measurements and experimental study (Calama-González et al 2018, Varela Luján et al 2019). Cho et al (2013) also stressed that the simulated thermal performance results should be compared through experiment, and applicability in the field should be investigated in future research. Apart from accuracy and verification of building simulations, it was further reported that occupancy patterns and occupancy behaviour should be studied in more detail in the future, since a lack of occupant and equipment loads lead to inaccuracies (Calama-González et al 2022). A similar research gap was also reported in another study which stressed the need to include the occupant into the design process as part of developing an effective social participatory research tool (Onyszkiewicz and Sadowski 2022). Other studies that investigate energy performance, also presented some research gaps when using case studies as a methodology. Gali Taşçi et al (2017) reported that building performance simulation case studies have limitations in location, direction, and surrounding buildings. Liu et al (2020) stated a similar limitation in reporting that building performance simulation tools fail to consider different urban morphology and the urban microclimate, and that this remains to be investigated in future research. Studies that investigated daylighting reported that modelling and computer simulations, and numerical simulations had certain limitations in accuracy and data availability and processing, and reported the need for future work to consider daylight performance in conjunction with thermal performance for enhanced energy efficiency (Noblejas et al 2021, Zhang et al 2022).

Among the studies that investigate building performance there were reported gaps on the study duration, where the thermal performance of green walls in tropical areas such as Vietnam (Nguyen et al 2019) and Singapore (Cheng et al 2010) was only studied for summer while identifying winter months as requiring future research. Furthermore, other researchers reported gaps in the area of humidity and hygrothermal performance, in particular how moisture can accelerate building defects and loss of thermal performance, and requiring further work to develop technical standards on hygrothermal performance requirements (Paiho et al 2019, Isiofia et al 2022, Shao et al 2022).

Numerous studies have highlighted a research gap that has overlooked the impact of climate change on thermal and cost performance. For instance, Yoon et al (2023) reported that changes in energy use due to climate change and associated energy cost increases need to be further studied. Likewise, Liu et al (2020) has concluded a research opportunity that involves cost-benefit analysis of passive design strategies in the context of climate change and related energy cost increases. In warmer regions, climate change is expected to significantly decrease indoor conditions and reduce thermal comfort. Therefore, some studies have reported the need to investigate future weather climate change scenarios, especially the impact of extreme heat occurrences on façade design and building performance (Calama-González et al 2018, 2022). Several studies have also stressed the need to consider future climate scenarios for developing guidelines that can improve future building regulations (Cuerda et al 2014, Diaz et al 2020, Eggenschwiler et al 2022). Apart from the research gaps in the domain of quantitative building performance and energy efficiency, researchers have also identified research gaps of a qualitative nature concerning human behaviour, human psychology, cognitive processes, individual preferences and cultural factors. Calama-González et al (2022) have reported a gap that lacks understanding of human behaviour which would have large impact on occupancy schedules and outcomes. Furthermore, two studies on daylight and thermal performance also mentioned that individual preferences and occupant behaviour were important deign consideration that should be further analysed in future research (Lu et al 2016, Zhang et al 2022). Another research area focused on aesthetic perception, reported study gaps in the evaluation of aesthetic parameters. Ilbeigi et al (2019) stated the need to study cognitive differences where cultural discrepancies of different education levels and nations could change the way we perceive and interpret the façade. Additionally, Azemati et al (2020) and Alkhresheh (2012) have reported that the impact of cultural background, individual preferences, gender preferences, climatic conditions and age limitations are important qualitative parameters that should be analysed in future work.

The geographical distribution of the study locations in the 105 selected articles is shown in figure 4 in which each dots represents one study location. To facilitate the analysis, the Köppen–Geiger climate classification system was simplified into 5 climate types (see figure 4), namely Tropical (A), Dry (B), Temperate (C), Continental (D) and Polar (E). Figure 6 shows data regarding the geographical distribution, 51 studies (48.6%) are conducted for a Temperate climate in Mediterranean countries or European countries, 18 studies (17.1%) in a hot and Arid climate, whereas only 15 (14.3%) and 10 studies (9.5%) were conducted for Tropical and Continental climates, respectively. In terms of urban context, it can be observed in figure 7 that the building heights of the selected studies are predominantly low rise with 53 studies (50.5%) being 1–4 storeys, 13 (12.4%) medium rise (5–9 storeys), and only 8 (7.6%) high rise (10 storeys or above). Regarding the distribution of studies, it is evident that there is a lack of studies for the tropical climate, and for the high-rise building typology. These two research gaps coincide with global urbanization trends.

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Since population growth mainly occurs in areas with a tropical climate such as South Asia and Africa (Ritchie and Roser 2018), and high-density living will become more common (United Nations 2018), there is a justifiable need to investigate residential buildings from this perspective. The urban population has grown exponentially since the latter half of the previous century. In the 1950s the urban population was below 200 million. At the turn of the century, this increased to 3 billion (UNFPA 2006). By 2030 it will reach 5 billion and by 2050, it is estimated to surge to 9.2 billion (UNPF 2022). 80% of the global population growth from 1990–2010 has occurred in urban areas mainly located in Asia and Africa (Ritchie and Roser 2018). The UN recorded 33 megacities with more than 10 million inhabitants, 48 cities with 5–10 million, 467 cities with 1–5million and 598 cities with 0.5–1million (United Nations, Population Division 2018). Most of these megacities are situated in regions with a tropical climate. In response to these urbanization phenomena, more high-density cities—an inevitable path of modern civilization—have emerged. The results suggest that there is a lack of studies considering the building performance of high-rise buildings in the tropical and subtropical climate in a high-density urban context. Furthermore, it is necessary to investigate this gap in combination with the impacts of climate change, which can have significant implications for the urban microclimate, especially in tropical climates where excess heat is exacerbated by climate change.

Moreover, global urbanisation trends also indicate that high-density cities associated with an increase in high-rise buildings will become more common, especially with increasing megacities mainly concentrated in the tropical and subtropical regions of Asia and Africa. Additionally, climate change inevitably causes global warming. Some cities with existing temperate climate will likely witness a transition to a warmer subtropical climate, or experience more frequent and intense heat waves. It is, therefore, important to conduct further research on how residential façades should be designed in a dense urban context, especially for high rise buildings in hot and humid climates. Furthermore, as a high-density urban context will become more prevalent with an increasing number and height of buildings, that will affect solar access and ventilation corridors. It will become even more crucial to consider their impact on the urban microclimate in the future.

While there has been substantial research on the quantitative aspects of building performance, the study has pointed out a lack of research on the impact of future weather scenarios including climate change and other elements with a certain degree of uncertainty such as human behaviour, individual habits, and cognitive processes. Although climate change and occupant behaviour are challenging to accurately predict, these must be considered for building performance. How people use a space, and when people use a space has significant implications on the occupancy schedule which in turn affects the accuracy of building performance simulations. Furthermore, buildings that are designed for the present need to be equipped and adapted for the future, where it is only logical that future weather datasets must take precedence over current weather datasets.

There is a need to critically evaluate future design consideration with respect to climate change. Global warming will inevitably lead to rising temperatures. This in turn will lead to an increase in energy consumption due to prolonged use of air-conditioner to maintain comfortable indoor thermal conditions. The design of an optimised flat and façade design would include multiple user scenarios with varying household behaviour. Several studies state that household behaviour has a great impact on energy use, up to 80% or more for any residential apartment (Deng and Chen 2019, Duan et al 2022). Therefore, making the design adaptive for both the shoulder season, and peak summer season would be crucial in addressing different user requirements and behaviour.

Regarding the shoulder season which lasts from around April to May, and September to October, suitable passive design strategies can increase indoor thermal comfort and greatly reduce the need for AC cooling. Considering global warming, it will be important to implement design strategies which can lengthen the shoulder period. The use of AC cooling will be necessary to maintain acceptable indoor thermal comfort during increasingly hot summer months. Thus, it is necessary to design the flat and the façade in such a way that cooling loads can be minimal during peak season. Designing a flat for both the shoulder and peak summer season can be challenging. Passive strategies that lengthen the shoulder period, generally improve thermal comfort by enhancing natural ventilation, and therefore reduce the use of AC cooling. However, design strategies which improve indoor thermal comfort often result in increased AC cooling energy consumption during peak summer season. This is because improvement in natural ventilation conditions often sacrifices thermal insultation to some extent resulting in increased AC energy consumption especially during peak summer months. For example, a space with higher floor to ceiling height, and larger window to wall area (with openable windows) would improve natural ventilation and would improve thermal comfort during shoulder months. However, this would also lead to increased AC cooling consumption whenever the occupant uses AC.

Future residential design should investigate the potential benefits of designing an apartment according to household behaviour and different seasons.

In addition, other socio-economic factors are also important parameters that must be critically evaluated when designing for the future. Land scarcity and increasing real estate values, along with shrinking family size could lead to smaller average flat sizes. This trend is already observed in many developed countries. These are often characterized by an aging population with low birth rates and coupled with a more educated society. All these sociodemographic factors lead to a much smaller family size than in previous generations, and correspondingly a rise in demand for smaller flats. Smaller flats in the form of a self-contained studio where the kitchen, living room and bedroom are contained in a single space are problematic, as they do not allow for design adaptations. Furthermore, smaller flats also exhibit higher thermal sensitivity due to poor heat dissipation from anthropogenic heat generation such as using cooking stove, electronic equipment, or other types of latent heat.

Other important factors to consider are changing and evolving lifestyles. For example, Covid-19 has changed the way that people work, promoting the concept of work from home. It is also obvious that elderly citizens have a very different routine from working middle aged people who spend most of their day away from their homes. Future work may consider if there is an optimal flat size, and façade ratio, or division of space that could balance energy, cost, and comfort of various user groups. Furthermore, we may need to consider the ratio and relationship between the façade and the volume of the residential flat, in addition to climate change, and human behaviour.

Building regulations exists to protect the safety, health, and wellbeing of people while improving the conservation of fuel and energy, enhancing the environment, and promoting sustainable development. The aforementioned research directions may provide new insights for both researchers and policymakers, and ultimately assist in developing guidelines that can improve future building regulations. This requires a holistic understanding of what the future may look like and how buildings should be designed for mitigation, adaptation, and resilience. It is necessary to critically evaluate how scientific research on urban and building climatology can be translated into practical implementation that can have a wider impact. Essentially, building regulations present an invaluable tool that can improve the implementation of research findings through legal enforcement.