What Is Your Building Doing for the City? Systematic Literature Review on the Potential of Façade Design for the Mitigation of Urban Environmental Problems

Abstract

Rising urban temperatures, noise and air pollution, and the loss of biodiversity are pressing problems in cities worldwide that call for action at different scales to improve the livability of urban areas. This study focuses on the role that buildings and façade design play in the urban environment, exploring how their informed design might help mitigating these environmental issues at a local scale. It explores the field by means of a systematic review aimed at identifying the impact of façade design choices focusing on three main design variables: material, geometry, and vegetation in façades. Scopus and Web of Science databases were explored between 17 April and 20 April 2023, ending up with 121 scientific articles, then categorized and data-mined to allow for descriptive statistical analysis to discuss scientific results obtained via digital simulation or empirical measurements. Risk of bias was addressed through double revision of the gathered sample. This study ends with the identification of desirable façade attributes based on their reported impacts, in terms of material properties, geometric operations, and main vegetation parameters, which we discuss by outlining compatibilities and clashes between them to guide conscious building design decisions to improve the urban ecosystem.

1. Introduction

More than 50% of the world’s population lives in urban areas, and this number is expected to increase to 68% by 2050 [1]. Although urbanization processes aim at improving health, social, and economic conditions for the population, the exponential growth of urban areas has generated a series of environmental problems that not only threaten local and global ecosystems but have been largely reported as serious health hazards for their inhabitants [2,3].

Studies have shown that prolonged exposure to polluted air reduces life expectancy [4], while short-term exposure has been related to higher daily mortality rates in cities [5], an increase in respiratory diseases, and the prevalence of asthma in children [6]. Likewise, it has been shown that urban noise causes anxiety and annoyance, which, added to other design variables of cities, generates alterations in the mental health of their inhabitants [7,8]. In this regard, some studies have established a relationship between growing up in cities and the presence of greater risks of suffering from schizophrenia [9,10]. Regarding the high temperatures associated with urban heat islands, studies have reported increases in cardiovascular and respiratory mortalities during heat waves [11], estimating that the mortality rate associated with heat in the elderly doubles with every degree increase in the ambient temperature [12].

Furthermore, cities are responsible for two thirds of the global energy consumption, and over 60% of greenhouse gas emissions [13]. In addition, the accelerated growth of unplanned urban areas has been pointed as one of the greatest threats to the biodiversity of local ecosystems [14,15], a situation that will be exacerbated by the increasing effects of climate change. In this sense, the latest report by the Intergovernmental Panel on Climate Change [16] indicates that cities are particularly vulnerable to impacts derived from this, reinforcing the urgent need to apply adaptation strategies to improve the well-being of their inhabitants while safeguarding and ideally regenerating the local ecosystem, following a systemic approach to tackle the problem.

Different alternatives have been discussed in scientific and political forums, considering general calls for greater sustainability in the built environment [17], the inclusion of nature-based solutions in the planning of our cities [18,19], and the application of mitigation and adaptation strategies to climate change, under a regenerative paradigm [16,20]. Thus, it is not only about reducing negative externalities or striving for an efficient use of materials and energy resources but about actively improving the urban ecosystem with each intervention, seeking positive impacts at different scales [21,22,23].

It is within this search for positive impacts that building envelopes are identified as relevant players in the urban ecosystem, not only because of their key role in mediating the exchange of energy and matter flows between the interior and exterior of buildings but also due to their role in defining and qualifying public spaces [24,25]. Hence, the (re)design of building envelopes presents a clear opportunity to generate small-scale environmental improvements, following an “urban acupuncture” strategy for local regeneration, in parallel to large urban transformations of high cost and complexity in their management [26]. This approach seems especially relevant in high-density urban contexts, such as metropolitan centers, where the urban fabric is largely consolidated without leaving much space for major interventions. However, it is crucial to carefully estimate the potential impacts that building envelopes may have on their surroundings, to support their responsible design and promote their interventions on the existing fabric.

In recent years, an increasing number of studies have explored the impact of building envelopes on distinct urban environmental issues such as their localized effect on urban heat islands [27,28] or their impact on the urban soundscape [29]. Likewise, the possibility of using the façade to clean polluted air or serving as support for local flora and fauna has been advocated as regenerative strategies for the urban ecosystem [30,31]. These studies are regarded as evidence of the increasing interest in exploring this path, fostering further understanding of previously unforeseen impacts of buildings and façade design. Nonetheless, while highly valuable, these are still regarded as isolated cases of research in specific fields, describing at best identified impacts associated with a building envelope under specific scenarios concerning one environmental domain. Therefore, a holistic approach is needed to support conscious building design choices that lead to the betterment of urban spaces facing simultaneous environmental problems.

This paper contributes to that regard by presenting a panorama of current knowledge on these fields, based on a systematic literature review of scientific articles from online databases. The review aims at (i) generating a comprehensive overview of state-of-the-art studies, by describing and organizing the reported impacts of building envelopes on selected urban environmental issues and (ii) discussing the reported impacts and identifying desirable façade design attributes to cope with the selected issues, while showcasing potential clashes between diverging requirements and responses. The review was conducted under the following framing questions:

• What research is being conducted regarding the impact of the design of building envelopes on urban environmental problems?
• What impacts on the urban environment have been reported regarding different façade design aspects?

To frame the scope of the literature review, it was decided to focus on four urban environmental issues, widely accepted as serious challenges that currently besiege urban areas: (i) urban heat, (ii) air pollution, (iii) noise pollution, and (iv) biodiversity loss.

2. Materials and Methods

The systematic literature review was conducted following the PRISMA statement, a protocol developed for the transparent reporting of systematic reviews and meta-analyses in 2009, and later updated in 2020, consisting in an evidence-based minimum set of items for reporting results from scientific sources [32]. The PRISMA protocol follows three main stages for the systematic revision of potentially relevant scientific reports: (i) identification, (ii) screening, and (iii) included reports, requiring detailed and transparent information at each stage. The PRISMA flowchart for this study is presented in Figure 1, with detailed information at each stage described in the following sub-sections. Additionally, the PRISMA checklist for this article is available in Supplementary Materials.

2.1. Identification of Relevant Scientific Reports

First, it was decided to conduct the review on Scopus and Web of Science scientific databases, arguably the most relevant and respected academic databases in the field. As a starting point, a series of exploratory search queries were conducted between 17 April and 20 April 2023 to attune and define the search parameters, thus ensuring relevant results while keeping the matches at a reasonable number to allow for their detailed review. We performed the queries on both databases, considering matches on the title, abstract, or keywords, which are presented in

Table 1. Search parameters and results of the queries. The search string used for the review is highlighted in the table.

Search Queries (Title, Abstract, Keywords)		Scopus		Web of Science
Topics	Search String	Results	Date	Results	Date
Noise	(façade OR envelope) AND noise	11,150	18.04.23	5017	20.04.23
Noise	(façade OR envelope) AND noise AND urban	433	18.04.23	198	20.04.23
Noise	(façade OR envelope) AND noise AND urban AND design	121	18.04.23	60	20.04.23
Biodiversity	(façade OR envelope) AND biodiversity	500	18.04.23	591	20.04.23
Biodiversity	(façade OR envelope) AND biodiversity AND urban	52	18.04.23	42	20.04.23
Biodiversity	(façade OR envelope) AND biodiversity AND urban AND design	20	18.04.23	16	20.04.23
Heat	(façade OR envelope) AND heat	12,358	18.04.23	8495	20.04.23
Heat	(façade OR envelope) AND heat AND urban	840	18.04.23	721	20.04.23
Heat	(façade OR envelope) AND heat AND urban AND design	296	18.04.23	277	20.04.23
Heat	(façade OR envelope) AND “urban heat”	405	18.04.23	340	20.04.23
Heat	(façade OR envelope) AND “urban heat” AND design	127	18.04.23	112	20.04.23
Air pollution	(façade OR envelope) AND “air pollution”	1051	18.04.23	266	20.04.23
Air pollution	(façade OR envelope) AND “air pollution” AND urban	197	18.04.23	103	20.04.23
Air pollution	(façade OR envelope) AND “air pollution” AND urban AND design	48	18.04.23	19	20.04.23

.

As Table 1 shows, both terms “façade” and “envelope” were considered, as they are commonly used interchangeably in the specialized literature. Then, we coupled these terms with the defined urban environmental issues shown in the column labelled “Topics”, adding the term “urban” to focus on the urban impact instead of indoor performance and “design”, when needed, to narrow down and limit the number of results under 150 matches per topic. Given the large number of matches under “heat” as a stand-alone term, it was decided to use “urban heat” as a compound term instead, aiming for articles tackling the impact of façades on the external thermal environment and localized urban heat islands. The final search string used for the review is highlighted in the table.

The final search string for the four defined topics resulted in 348 and 233 matches in the Scopus and Web of Science databases, respectively. These matches were compiled, and 137 duplicated results were removed from the process, resulting in 444 articles to be screened.

2.2. Screening of Relevant Reports and Inclusion/Exclusion Criteria

First, 35 conference reviews and non-English articles were excluded. Then, of the 409 reports sought for retrieval, 85 documents were not available, resulting in 324 full reports to be assessed for eligibility, within the boundaries defined for the literature review. These reports were carefully checked to see if they met the required criterion to answer the research questions, namely, assessing if the specific focus of the studies explicitly addressed direct impacts of façade design on each defined environmental issue at the local scale. The use of this criterion meant that articles that reported on (i) indoor impacts of façade design, (ii) urban or territorial scale strategies to deal with environmental issues, or (iii) general statements regarding the defined topics were excluded. Similarly, we also excluded articles that explored (iv) potential indirect impacts of building design on the defined environmental issues, striving for focused studies on explicit direct reported impacts.

The eligibility assessment was first conducted by M. Pastén and then checked independently by A. Prieto. Afterward, we discussed inconsistencies and rechecked the cases in doubt in further detail. At the end of the process, a list of 121 reports was compiled as the basis for the review.

2.3. Reports Included in the Detailed Review

Of the 121 selected reports, 95 correspond to original articles, 19 to conference papers, and 7 to scientific reviews. Early in the screening process, given the still exploratory state of the topics at hand, it was decided to keep conference papers and review papers if they fell within the defined inclusion criteria.

The assessment of the articles followed a two-step approach, following the framing of questions defined for this study. First, the articles were assessed and categorized according to the urban environmental issue they tackle, the methods they employ, and the main façade design variables they explore to build a panorama of the research in the field as an answer to the first question under bibliographic assessment. The second step considered the detailed revision of the reported findings from the studies to identify impact indicators and discuss the role of the selected design variables on the urban environmental issues. The detailed revision was primarily carried out by M. Pastén and independently revised by A. Prieto. We checked for the impact when stated in the articles to synthetize and discuss the findings through descriptive statistical methods, presented in graphs, charts and tables. Only articles with identifiable impact indicators are discussed and explicitly referenced in the text to answer the second research question, amounting to 79 scientific reports.

3. Results

3.1. Façade Design and the Urban Environment

Figure 2a shows the number and type of articles considered in the review per urban environmental issue, as a result of the systematic screening process. Out of the 121 articles that responded to the defined criteria, 66 address the role of the building envelope regarding urban heat, 31 noise issues, 12 air pollution, and 12 the lack of biodiversity. Based on the number of matches, it is evident that the relation between the building envelope and the heat and noise urban environmental domains has been more studied, in comparison to the impact of the building envelope on air pollution and biodiversity, which remains largely incipient. This statement is reinforced by Figure 2b, which shows the number of articles per year. The graph shows that almost all articles considered in the review were published in the last decade, serving as testimony to the relative novelty of the general focus. Nonetheless, studies about the impact of façades on biodiversity and air pollution have remained scattered, while there has been a clear increasing research output concerning noise and urban heat issues, particularly in the latter.

3.2. Reported Impact of Selected Façade Design Variables on Urban Environmental Issues

The selected articles were reviewed and primarily assessed in terms of the reported impact of façade design variables on each defined urban environmental issue. We identified and contrasted these reported impacts to discuss the role of the selected design variables in mitigating the issues, considering the location of the experiment and the methods applied in the experimental setup to provide the context of the discussion for a fair assessment of the results. Figure 3 shows the main research methods considered, where “monitoring” stands for the use of sensors to obtain data from real cases or in laboratory setups, and “simulation” comprises both static numerical modeling and dynamic simulation via specialized software. Lastly, the articles categorized under “review” focus on the analysis of information from secondary sources, namely previously reported scientific data.

Furthermore, we opted to categorize the reported impacts regarding three distinct types of façade design variables largely considered in the literature: geometry, material, and vegetation (Figure 4). Accordingly, the assessment of the results is presented on these terms, aiming at identifying and discussing the potential impact of the surface geometry, material composition, or the presence of vegetation on the mitigation of the defined urban environmental issues.

3.2.1. Reported Impact on Air Pollution

The impact of the envelope on air pollution had the lowest number of cases matching the search criteria with only 12 papers. Simulations and scientific reviews were the main research method in five articles each, while only two cases considered monitoring of prototypes under laboratory conditions [33,34]. The articles discuss impacts of wind currents in urban canyons [35] and the application of nature-based solutions to façades, such as vegetation, microalgae, and water [36,37], on air pollution removal and the integration of technical equipment such as cyclone collectors, wet scrubbers, or electrostatic precipitators [36,38,39].

Geometry

Articles that discuss the impact of geometry on air pollution focus exclusively on the role that building surfaces play on wind behavior in dense urban contexts, which indirectly affect the dispersion of pollutants in cities. Di Sabatino et al. [40] studied the behavior of air fluxes in urban canyons through simulations, concluding that street canyons with an aspect ratio of one (height: width) generate vortexes closer to the street level, while increasing the height of the buildings results in vortexes closer to the top of the building. Similarly, it has been reported that when the windward wall and ground are warmer than the leeward side, a second, smaller vortex forms, splitting incoming currents into two counterflows, which support an easier dispersion of contaminants in the canyon [33,41].

The effect of balconies on washout rates was explored by Murena and Mele [42], reporting that the presence of continuous horizontal balconies promotes slower recirculation zones compared to balconies that are located separately throughout the façade, particularly in its lower areas. Hence, greater air mass transport occurs in urban canyons where balconies are separated, although there appear to be no discernible differences when the separation is larger than 1 m. Finally, air recirculation and washout rates always appear to be higher in the upper areas of the canyon as expected [43].

Material

There is no relevant information reported in the surveyed sample regarding the impact of façades’ material choices on air pollution in urban settings.

Vegetation

Regarding the implementation of vegetation on roofs and façades, it was reported that the use of a green wall in street canyons with aspect ratios of one and two (height/width), could lead to reductions of up to 35% in NO₂ concentration and up to 50% in PM₁₀ concentration [44,45] if a careful selection of species is conducted, based mostly on their leaf area index (LAI). Regarding the relation between façade height and effectiveness, no variations in particle depositions were reported for different heights of green walls near a traffic corridor [46].

Di Sabatino and Barbano [40] explored the relationship between urban heat island intensity and pollutant concentration via Computational Fluid Dynamics (CFD) models, reporting a strong correlation and arguing for the use of vegetation to positively tackle both issues. Along the same lines, the implementation of green roofs in street canyons was reported to result in a 32% reduction in pollutant concentrations, with a reduction of 2 °C at the breathing level. Further calculations have showed that 1000 m² of green roofs could potentially capture 160–220 kg of dust per year. Hence, 1 m² of green roof would be enough to offset the annual particulate matter emission of one car. [37].

3.2.2. Reported Impact on Biodiversity

The potential impacts of façades on biodiversity are still largely unexplored in the specialized literature, evidenced not only by the 12 search matches, but also by the fact that most studies refer to qualitative aspects rather than quantitative indicators when reporting impacts, hinting at the lack of metrics to comprehensibly understand and evaluate the phenomenon.

Geometry

Two articles explore the role of façades’ geometry on biodiversity in cities. Kensek and Ding [47] examined the impact that buildings have on birds in urban settings, focusing specifically on geometrical and material choices that increase bird collisions. Hence, the authors assessed the bird threat level of common façade materials and analyzed building envelope geometries relative to these materials, then to program a software tool compatible with common building information modelling software to automate the assessment and provide input to the design team during early design stages. The second article focused on the resulting geometries from the design of green walls where the crops are arranged randomly [48]. The arrangement has an impact on the image of the façade and the successful growth of different species, since varying geometries may create unwanted self-shading in some areas, limiting sunlight exposure. This may affect each plant species in different ways, so the determination of correct crop ratios and the resulting geometries will vary according to species selection, quantity, and arrangement, which need to be thoroughly considered.

Material

Besides the previously mentioned article by Kensek and Ding [47], which explored the role of façade material choices on potential bird collisions, other articles focused on material aspects of the building envelope. Davidová [49] conducted revision of vernacular architecture solutions, particularly focusing on envelope constructions and screen walls as the main elements that may support the biodiversity of a particular environment, bridging the gap between human production and its surroundings. Construction systems based on earth, wood, the use of thermal mass in mediterranean zones, or permeable fine elements in northern countries are cited as examples in a collection of thermal and climatic parameters, which were then applied in the design of a prototype aimed at serving as habitat for bees and bats. Meier and Raps [50] focused on the thermal characteristics of materials for the assessment of a wall habitable by insects. The authors analyzed the thermal conductivity and fire resistance of the proposed prototype in comparison to common wall constructions and external thermal insulation composite systems (ETICSs). Furthermore, it was reported that the abundance and richness of species increases in the presence of high moisture, which is posed as a relevant challenge to technically integrate in the design of wall constructions that aim at supporting biodiversity in urban settings [31].

Vegetation

Reports of quantitative data linked to biodiversity in the surveyed articles correspond exclusively to the experiments on the integration of plants and vegetation in building envelopes. In these research experiments, manual instruments aimed at collecting quantitative information regarding the abundance and richness of insect species in a determined area of the building enclosure were used [31,51,52]. Nevertheless, most reviewed articles on the subject followed a theoretical approach, aimed at identifying links between diverse species needs and specific environmental variables based on extensive case studies, secondary sources, or previous experiments [48,49,53], while some aim even further by proposing a framework to support the design of living walls and green façades under an ecology-based holistic design process [54,55].

3.2.3. Reported Impact on Noise

The collected articles that reported on the impacts of the building envelope on urban noise amounted to 31, a number considerably higher than the numbers matching the previously discussed environmental domains. Of this total, six were review papers and 25 consisted of reports on original research, whose key findings are compiled and showcased in Table 2. The research methods applied mostly correspond to computer simulations via raytracing models using specialized software such as ODEON (v.17) [56,57], other than instances of reported experimental measurements of sound pressure levels, reverberation time, or sound absorption under laboratory conditions or monitoring campaigns in real urban canyons or courtyards.

Geometry

Regarding the general impact of building geometry, studies reveal that lower building heights and wider streets conclusively result in reduced traffic noise in the street [58,59,60]. Similarly, it has been reported that a decrease in the aspect ratio of urban canyons (height: width) changes acoustic comfort by attenuating sound pressure levels by reducing reverberation times along the canyon [61]. It was consistently reported that reverberation in upper floors is markedly higher in enclosed urban scenarios, compared to open urban forms [62]. As expected, not only increasing the distance between façades alongside a city street but also the presence of gaps between buildings have been found to reduce noise at the pedestrian level, favoring isolated constructions instead of continuous urban canyons when it comes to acoustic comfort [57,59,62].

Table 2. Noise mitigation key findings.

Ref.	Design Variables	Research Methods	Key Findings
[63]	Geometry Material	Computer simulations	Balconies reduced the average sound pressure levels (SPLs) on the 3rd floor by 2 dB but have minimal impact on lower floors. Absorbing materials on balcony ceilings helps reduce floor-level noise. Increasing balcony depth from 0.9 m to 1.5 m reduced the mean SPL by 1 dB on the case-study façade. The inclination of the parapets and inferior surfaces of the balconies increased the mean SPL up to 1 dB. Improved cladding materials significantly reduced the average SPL on both studied façades by 1 dB.
[56]	Material	Computer simulations	Sound absorption coefficients of façades have a visible influence, while scattering coefficient of façades have an influence only when a reflecting material is applied.
[64]	Vegetation Material	Experimental model	Sound absorption effects of introducing a 2.5 cm air gap and perforation in panels were studied. Higher perforation rates corresponded to a higher absorption frequency range. Panel absorbers and perforation structures improved the sound absorption at frequencies of less than 1000 Hz.
[62]	Geometry	Computer simulations	Multiple cross-reflections increase the SPL by about 6 dB in urban canyons and by more than 11 dB in courtyards, compared to an open-view referential building. Increasing the distance between façades reduces the sound level at the receiver due to a higher probability of the ray to be reflected toward the sky.
[65]	Geometry	Computer simulations	Balcony design has a great influence on façade noise levels. inclining the balcony ceiling and ledge in upper floors and adding absorption on the ceilings up to the 3rd floor significantly reduced the average noise levels along windows in upper floors up to 12.9 dB(A). The addition of triangular prominences on the façade had a strong influence on the façade exposure, especially at the upper floors, up to 8.4 dB(A). Self-shielded windows provided important noise reduction along the façade. Reduction up to 7.6 dB(A) in the upper floors is achieved. Reduction was proportional to the angle of inclination.
[66]	Geometry Material	Computer simulations	It is beneficial to cover façades with sound absorbing materials to reduce the SPL, thus reducing noise annoyance. Diffusing façades are strongly linked to noise annoyance.
[67]	Geometry Material	Computer Simulation and Experimental Model	Standard louvres tend to provide lower sound protection to the building façade, with respect to the sound absorbing shading system. In general, bigger louvres protect the building façade more from road noise. Louvre size is more important on higher floors because of the decrease in the angle of incidence. Wider distances between louvres give poorer sound protection to the building façade. Traditional louvres increase SPLs by up to 4 dB on the opposite building façade, with some reduction by sound-absorbing shading devices. On higher floors, the reduction in SPL ranges from 2 dB to 3 dB(A).
[68]	Vegetation Material	Computer Simulations and Experimental Model	Noise levels before and after incorporating vegetated walls were measured. The results suggest that noise reduction of less than 2 dB(A) can be achieved in a two-lane street canyon.
[69]	Vegetation Geometry Material	Experimental model	Vegetated façades provided 1.6 dB(A) of noise reduction in the street canyon, and 2.6 dB at frequencies below 1 kHz. Vegetated façades with shrubs provided the same noise reduction as vegetated façades alone. In the courtyard, the green roof system reduced noise diffraction over 1 kHz by up to 10.9 dB, while the noise reductions below 630 Hz were lower than those of the vegetated façades in the courtyard. Vegetated façades with no green roof led to similar noise reductions of approximately 2.5 dB(A).
[59]	Geometry Material	Computer simulations	Lower building height and wider streets result in SPL decrease from traffic noise. A gap between buildings could provide about 2–3 dB extra sound attenuation Absorption via air and vegetation is effective in attenuating noise along the street, with an extra effect between 3 and 9 dB at relatively high frequencies.
[70]	Vegetation Material	In situ measurement	Replacing reflective boundaries with diffuse ones in urban streets increases sound attenuation by 4–8 dB over 60 m and reduces reverberation time by 100–200%. Windows with two or more layers of glass could bring benefits in both energy saving and noise reduction. Micro-perforated absorbers (MPAs) may be used along the path created to reduce noise.
[71]	Vegetation Geometry	Computer Simulations and in situ measurement	Ivy increases extra SPL attenuation with distance at mid and high frequencies, up to 5.2 dB at 4000 Hz, but negative effects occur at low frequencies due to Ivy’s high scattering coefficient. Green walls could reduce sound levels by 9.9 dB at 500 Hz and speech levels by 9.3 dB(A). For green walls, extra SPL attenuation increases with distance at all frequencies. Porous growing media is recommended over just leaves for better sound absorption/scattering at high frequencies. Combined use of vegetation on the ground and the façade showed that façades played the larger role in noise reduction.
[72]	Geometry Material	Experimental model	A good measure for noise reduction is the upward horizontal tilting and more repetition and thickness of eaves on the façade in case of airplane noise (source from above).
[73]	Material	Computer simulations	The differences in the acoustic effect between normal exterior plastic and highly absorbing material can be approx. 6 dB.
[60]	Material	Computer simulations	The reduction of SPLs over building height is more pronounced. The use of absorbent plaster on a façade in an urban canyon of 12 m width provides an average reduction by 1.5 dB(A) in a receiver at h = 1.5 m and up to 3.3 dB(A) in a receiver positioned at h = 28.5 m.
[74]	Vegetation	Computer simulations and in situ measurement	Vegetation and its soil substrate offer a way of increasing absorption and/or diffusivity in an urban setting. The maximum reduction from green façades was below 2 dB. Façade vegetation on lower floors resulted in limited impact. Full courtyard greening is more effective than partial. Green roof geometry influences insertion loss in canyon street.
[75]	Geometry	Computer Simulation and Experimental Model	In the case of the inclined type eaves, an incident sound is blocked by the lower eave, and it is reflected outward from the façade of the building by the upper eave. In the case of louvres, the direct sound is blocked by small fins, but many fine reflections occur. The noise reduction effect is higher for receiving points located higher throughout the building.
[57]	Geometry Material	Computer Simulation and Experimental Model	Moderate façade absorption was found to increase acoustic comfort, and both reflective and too absorptive façades were associated with low acoustic comfort ratings. Hence, applying materials with moderate absorption characteristics in façades or using highly absorbing materials for only a selected portion of the façade could be useful. Absorbing materials on balcony ceilings tended to increase acoustic comfort on balconies.
[76]	Vegetation Geometry Material	Computer simulations	The height/width urban canyon ratio has been found to influence noise attenuation along the canyon. Gaps between buildings can provide about 2–3 dB extra sound-level attenuation. The effect of roof shape minimizes noise levels at the shielded side of buildings. The SPL decreased by increasing the scattering coefficient. Acoustically absorbing balcony façades were found to be either significant or insignificant based on their position and angle. Reflecting façade materials and acoustically absorbing materials on the balcony ceiling reported increased acoustic comfort.
[77]	Vegetation Geometry Material	Computer simulations	Green roofs have the highest potential to enhance quietness at indirectly exposed façades and in courtyards. Favorable combinations of roof shape and green roofs were identified, resulting in reductions up to 7.5 dB(A) in courtyards. Façade vegetation leads to most road traffic noise reduction when the materials of the building across are reflective.
[78]	Vegetation	Experimental model	With increasing vegetation coverage, the absorption coefficient is gradually increased. The scattering coefficient increases with higher vegetation coverage for all three plant species studied. The absorption coefficient gradually decreases with an increasing substrate moisture content, especially at low frequencies.
[79]	Geometry	Computer simulations	While the distance (m) increased between the site and elevated roads of 0 m, 100 m, 300 m, 600 m, and 1000 m, noise decreased

Table 2. Noise mitigation key findings.

Ref.	Design Variables	Research Methods	Key Findings
[63]	Geometry Material	Computer simulations	Balconies reduced the average sound pressure levels (SPLs) on the 3rd floor by 2 dB but have minimal impact on lower floors. Absorbing materials on balcony ceilings helps reduce floor-level noise. Increasing balcony depth from 0.9 m to 1.5 m reduced the mean SPL by 1 dB on the case-study façade. The inclination of the parapets and inferior surfaces of the balconies increased the mean SPL up to 1 dB. Improved cladding materials significantly reduced the average SPL on both studied façades by 1 dB.
[56]	Material	Computer simulations	Sound absorption coefficients of façades have a visible influence, while scattering coefficient of façades have an influence only when a reflecting material is applied.
[64]	Vegetation Material	Experimental model	Sound absorption effects of introducing a 2.5 cm air gap and perforation in panels were studied. Higher perforation rates corresponded to a higher absorption frequency range. Panel absorbers and perforation structures improved the sound absorption at frequencies of less than 1000 Hz.
[62]	Geometry	Computer simulations	Multiple cross-reflections increase the SPL by about 6 dB in urban canyons and by more than 11 dB in courtyards, compared to an open-view referential building. Increasing the distance between façades reduces the sound level at the receiver due to a higher probability of the ray to be reflected toward the sky.
[65]	Geometry	Computer simulations	Balcony design has a great influence on façade noise levels. inclining the balcony ceiling and ledge in upper floors and adding absorption on the ceilings up to the 3rd floor significantly reduced the average noise levels along windows in upper floors up to 12.9 dB(A). The addition of triangular prominences on the façade had a strong influence on the façade exposure, especially at the upper floors, up to 8.4 dB(A). Self-shielded windows provided important noise reduction along the façade. Reduction up to 7.6 dB(A) in the upper floors is achieved. Reduction was proportional to the angle of inclination.
[66]	Geometry Material	Computer simulations	It is beneficial to cover façades with sound absorbing materials to reduce the SPL, thus reducing noise annoyance. Diffusing façades are strongly linked to noise annoyance.
[67]	Geometry Material	Computer Simulation and Experimental Model	Standard louvres tend to provide lower sound protection to the building façade, with respect to the sound absorbing shading system. In general, bigger louvres protect the building façade more from road noise. Louvre size is more important on higher floors because of the decrease in the angle of incidence. Wider distances between louvres give poorer sound protection to the building façade. Traditional louvres increase SPLs by up to 4 dB on the opposite building façade, with some reduction by sound-absorbing shading devices. On higher floors, the reduction in SPL ranges from 2 dB to 3 dB(A).
[68]	Vegetation Material	Computer Simulations and Experimental Model	Noise levels before and after incorporating vegetated walls were measured. The results suggest that noise reduction of less than 2 dB(A) can be achieved in a two-lane street canyon.
[69]	Vegetation Geometry Material	Experimental model	Vegetated façades provided 1.6 dB(A) of noise reduction in the street canyon, and 2.6 dB at frequencies below 1 kHz. Vegetated façades with shrubs provided the same noise reduction as vegetated façades alone. In the courtyard, the green roof system reduced noise diffraction over 1 kHz by up to 10.9 dB, while the noise reductions below 630 Hz were lower than those of the vegetated façades in the courtyard. Vegetated façades with no green roof led to similar noise reductions of approximately 2.5 dB(A).
[59]	Geometry Material	Computer simulations	Lower building height and wider streets result in SPL decrease from traffic noise. A gap between buildings could provide about 2–3 dB extra sound attenuation Absorption via air and vegetation is effective in attenuating noise along the street, with an extra effect between 3 and 9 dB at relatively high frequencies.
[70]	Vegetation Material	In situ measurement	Replacing reflective boundaries with diffuse ones in urban streets increases sound attenuation by 4–8 dB over 60 m and reduces reverberation time by 100–200%. Windows with two or more layers of glass could bring benefits in both energy saving and noise reduction. Micro-perforated absorbers (MPAs) may be used along the path created to reduce noise.
[71]	Vegetation Geometry	Computer Simulations and in situ measurement	Ivy increases extra SPL attenuation with distance at mid and high frequencies, up to 5.2 dB at 4000 Hz, but negative effects occur at low frequencies due to Ivy’s high scattering coefficient. Green walls could reduce sound levels by 9.9 dB at 500 Hz and speech levels by 9.3 dB(A). For green walls, extra SPL attenuation increases with distance at all frequencies. Porous growing media is recommended over just leaves for better sound absorption/scattering at high frequencies. Combined use of vegetation on the ground and the façade showed that façades played the larger role in noise reduction.
[72]	Geometry Material	Experimental model	A good measure for noise reduction is the upward horizontal tilting and more repetition and thickness of eaves on the façade in case of airplane noise (source from above).
[73]	Material	Computer simulations	The differences in the acoustic effect between normal exterior plastic and highly absorbing material can be approx. 6 dB.
[60]	Material	Computer simulations	The reduction of SPLs over building height is more pronounced. The use of absorbent plaster on a façade in an urban canyon of 12 m width provides an average reduction by 1.5 dB(A) in a receiver at h = 1.5 m and up to 3.3 dB(A) in a receiver positioned at h = 28.5 m.
[74]	Vegetation	Computer simulations and in situ measurement	Vegetation and its soil substrate offer a way of increasing absorption and/or diffusivity in an urban setting. The maximum reduction from green façades was below 2 dB. Façade vegetation on lower floors resulted in limited impact. Full courtyard greening is more effective than partial. Green roof geometry influences insertion loss in canyon street.
[75]	Geometry	Computer Simulation and Experimental Model	In the case of the inclined type eaves, an incident sound is blocked by the lower eave, and it is reflected outward from the façade of the building by the upper eave. In the case of louvres, the direct sound is blocked by small fins, but many fine reflections occur. The noise reduction effect is higher for receiving points located higher throughout the building.
[57]	Geometry Material	Computer Simulation and Experimental Model	Moderate façade absorption was found to increase acoustic comfort, and both reflective and too absorptive façades were associated with low acoustic comfort ratings. Hence, applying materials with moderate absorption characteristics in façades or using highly absorbing materials for only a selected portion of the façade could be useful. Absorbing materials on balcony ceilings tended to increase acoustic comfort on balconies.
[76]	Vegetation Geometry Material	Computer simulations	The height/width urban canyon ratio has been found to influence noise attenuation along the canyon. Gaps between buildings can provide about 2–3 dB extra sound-level attenuation. The effect of roof shape minimizes noise levels at the shielded side of buildings. The SPL decreased by increasing the scattering coefficient. Acoustically absorbing balcony façades were found to be either significant or insignificant based on their position and angle. Reflecting façade materials and acoustically absorbing materials on the balcony ceiling reported increased acoustic comfort.
[77]	Vegetation Geometry Material	Computer simulations	Green roofs have the highest potential to enhance quietness at indirectly exposed façades and in courtyards. Favorable combinations of roof shape and green roofs were identified, resulting in reductions up to 7.5 dB(A) in courtyards. Façade vegetation leads to most road traffic noise reduction when the materials of the building across are reflective.
[78]	Vegetation	Experimental model	With increasing vegetation coverage, the absorption coefficient is gradually increased. The scattering coefficient increases with higher vegetation coverage for all three plant species studied. The absorption coefficient gradually decreases with an increasing substrate moisture content, especially at low frequencies.
[79]	Geometry	Computer simulations	While the distance (m) increased between the site and elevated roads of 0 m, 100 m, 300 m, 600 m, and 1000 m, noise decreased

Nonetheless, the role of façade geometry itself on noise abatement has been studied, particularly for cases where continuous urban canyons are unavoidable or even desirable, to respond to other considerations, such as the need for density, safety, or the construction of a unified urban image in city centers. In this regard, geometry has been studied for its potential to decrease noise reverberation by reflecting sound waves to the exit of a given urban canyon, which of course requires the accurate definition of the inclination angles for the geometry of the façade. The use of concave and triangular shapes was studied by Van Renterghem and Hornikx [77], who reported a reduction of 8.4 dB in noise levels on the upper floors of an urban canyon, but no discernible effects at the pedestrian level were registered. With the latter in mind, the use of high-reflective sound barriers with an inclination was studied near a noise source within a canyon by Echevarria Sanchez and Van Renterghem [65], who reported a traffic noise reduction of 8.8 dB.

Focusing on a smaller scale, the shielding effects provided by balconies, eaves, and louvres have been studied for the potential reflections that may add to acoustic comfort [60,77]. Studies have found that upward horizontal tilting in louvres and a higher repetition of eaves contribute to noise reduction, reporting a correlation between protruding elements in a façade and a reduction in sound pressure levels [61,72,75]. Also, as the building height increases, lower eaves can block the incoming sound towards the interior, reflecting it outside the building’s façade, which in turn may aggravate pedestrians’ acoustic comfort. Regarding louvres, they might reflect sound in multiple directions due to their small scale, which may help dispersing soundwaves [75]. Louvre sizes have been found to be more important on higher floors, offering protection since the angle of incidence from the traffic source decreases. As expected, it was reported that the wider the spacing between louvres, the lower the sound protection to the building will be [67]. In the case of balconies, it was reported that commonly used dimensions can reduce sound pressure levels up to 3 dB measured at the third-floor level; however, the use of complex geometries in façades could both mitigate or enhance noise discomfort if not properly designed considering the surrounding buildings or favoring the scattering of soundwaves, rather than diffusing or focally reflecting them [57,63].

Material

There is wide agreement over the use of absorbing materials in façades to reduce sound pressure levels in urban settings, being regarded as the most effective design measure to increase acoustic comfort for pedestrians and the inhabitants of the buildings [57,58,61,62,65,66,71,80]. Hence, porous materials are preferable compared to hard, reflective materials; however, the latter are more durable in widespread application, so a balance must be achieved by increasing the absorptive properties of high-performance materials, or carefully placing different materials according to zonal requirements. In this aspect, the combination of material and geometrical design choices has been studied to boost synergies for an integral response to noise. For instance, the use of absorbing materials in balcony ceilings has been reported as a useful measure to reduce noise at the pedestrian level [57,63].

Regarding common high-reflective materials in buildings, glass has been the focus of several studies. It has been reported that the low absorbing coefficient in glass (αw = 0.18) can be improved with double or triple glazing solutions if different glass pane thicknesses are combined with air gaps above 100 mm with absorbing material around the edge [81]. Similarly, in curtain wall assemblies, the implementation of Micro-Perforated Absorbers (MPAs) has been found to be beneficial in reducing noise reflection and transmission to the interior, but a relevant effect was reported only for noise frequencies lower than 1000 Hz [64,70].

In the case of courtyards, it has been found that material choice and the application of design measures in the surrounding façades and roofs can contribute significantly to the mitigation of noise from the road traffic outside the said courtyard. Regarding façade treatments, an increase in not only its absorption coefficient was reported to mitigate noise levels in the courtyard but also the diffusivity of the cladding, which circles back to both material and geometrical design choices for façades [82].

Vegetation

The sound absorbing properties of vegetation have been extensively studied in the surveyed sample of articles [58,59,64,68,69,70,71,78,80,82,83,84,85]. Thus, the controlled use of vegetation is regarded as an effective alternative to increase sound absorption in urban canyons, reducing multiple reflections and reverberation alongside street façades, especially achieving a reduction by 2–3 dB at high frequencies up to 5000 Hz [64]. The shielding impact of green façades is described to be related to acoustic properties of species, such as the leaf area and characteristics of stems and roots, but also the depth and density of the vegetated layer and moisture retention. Likewise, the substrate can be equally important in terms of noise reduction, especially supporting the absorption of high-frequency noises when it consists of porous soil media [83]. Plants with large leaves rooted in soil substrate were found to have an increased absorption coefficient at low frequencies, while also boasting a higher scattering coefficient at high frequencies. Moreover, it was found that the absorption coefficient can be gradually decreased in green walls by increasing the substrate moisture content, especially at low frequencies, reporting a total decrease of 0.2 [78]. Finally, green roofs were found to have the highest potential to enhance quietness at indirectly exposed façades and inner courtyards. Favorable combinations of roof shapes and green roofs were identified and assessed, leading to reductions of up to 7.5 dB(A) in confined courtyards [77].

3.2.4. Reported Impact on Urban Heat

Articles focused on the role of façade design on urban heat made up most of the reviewed sample, totaling 66 documents. This was expected given the wealth of information that has accumulated on the urban heat island phenomenon over the last decades, being cemented as a highly relevant environmental problem in cities worldwide. The reported studies consist of either the monitoring of real cases and prototypes in a controlled setup (n = 29) or digital explorations using numerical modelling or dynamic simulations via specialized software (n = 37). Moreover, the design strategies being explored in the reviewed sample appear to be quite balanced, considering the role of geometry, material, and vegetation on 26, 25, and 37 instances, respectively, with a higher relative amount for the vegetation.

The fact that this is a more explored field, with defined indicators, tools, and research methods, meant that clear quantitative impact data were showcased in some of the reviewed studies, presenting the opportunity for a deeper assessment of the results. Hence, the gathered articles were data-mined to compose a database to systematically explore and compare the reported impacts. The extracted impact data were normalized for their inclusion in the database in terms of temperature differentials, understood as the decrease in temperature achieved by each reported intervention, compared to each base scenario declared in the study.

The resulting database consisted of 656 data rows extracted from 36 articles out of the 66 initially selected for the review. The database comprises three distinct indicators used in the studies to assess the impact of the façade interventions on the thermal environment: External Surface Temperatures (ESTs), External Air Temperatures (EATs), and Mean Radiant Temperatures (MRTs). Most results referred to changes in the EST with 292 data points, followed by the EAT and the MRT with 210 and 154, respectively. These results were obtained either through monitoring real cases and experiments via sensors or through dynamic simulations via specialized software, as shown in Figure 5. The graph shows that the results for each indicator are highly dependent on the available methods, particularly in the cases of the EAT and the MRT, since it is harder to achieve consistent results on those indicators via monitoring due to contextual variables and the complexity of the sensing equipment on longer monitoring campaigns, especially in the case of MRT measurements.

A first glance at the results (Figure 6) shows that expectedly, the impact on the EST is more noticeable, achieving a temperature differential as high as −40 °C, with a median of 5.6 °C less than the base case scenario for each. The impact on the surface temperature was expected to be the highest out of the reported indicators, especially following the direct impact of incident solar radiation on façade materials with varying indices of heat conductivity and absorption. In turn, the heat accumulated and dissipated in these surfaces elevates the temperatures in their immediate surroundings, although with a lower relative impact compared to the EST. Hence, the decrease in the external air temperature (EAT) reported in the reviewed articles reaches a maximum of −4.9 °C in the local proximity of the evaluated façades; and this number increases up to −11.6 °C for the reported MRT values, evidencing a higher role of the surrounding surfaces’ temperatures, which are shown to be higher compared to the EAT. For both the EAT and the MRT, the reported medians are close to 0 °C, which points out the limitations of the expected effect that building surfaces could enforce on their thermal surroundings. Even though beneficial impacts are reported, these must be actively sought by the cautious application of design strategies following a careful consideration of local parameters.

Regarding the potential role of the design strategies discussed in this paper, Figure 7 shows the external surface temperatures (ESTs) reported in different articles before and after the application of a design measure aimed at heat mitigation. ESTs are chosen for this graph not only because more data were available but also because the broader range of impact results allowed for a clearer overview of the effect of the selected design strategies. The graph shows that the larger temperature reductions are obtained through the application of vegetation in the analyzed scenarios, followed by material changes on the base façades. The effect of geometrical changes appears to be more limited; however, a maximum decrease in temperature by around 20 °C was reported, evidencing the need to look at these strategies in more detail. Moreover, some studies reported negative results, obtaining higher ESTs after the intervention, whose boundary conditions need to be carefully assessed to thoroughly understand the impact of façade design strategies in their local thermal environment.

Geometry

Geometrical interventions in the reviewed studies had the lowest reported effect on external temperatures, especially considering its impact on the EAT and the MRT. Regarding the EAT, the simulated scenarios proposed by Peng and Jiang [86] via ENVI-met show a temperature differential ranging from 0 to −0.59 °C when comparing different urban forms (idealized urban blocks) with similar vegetation coverage. Similarly, Lan and Lau [87] reported values between −0.2 to +0.3 °C as a result of a series of simulations in the context of Hong Kong (June–August 2019), comparing a base case with a case where complex geometry was applied to enhance wind movement in buildings. The study also assessed the use of vegetation along with the geometric interventions, resulting in a slight increase of the reported overheating up to +0.7 °C compared to the base scenario. This was also reported by Di Sabatino and Barbano [40] after monitoring several points within a street canyon in Bologna during August 2017. The points were chosen to respond to varying heights and geometry, which were then compared to a meteorological station nearby. The results ranged from +0.5 to +9 °C, with an average of +5.1 °C, signaling that the air within the canyon was always warmer, with variations responding to daily oscillation and the sensor location.

These results slightly improve for MRTs, reaching up to −1.5 °C in temperature decrease in the mentioned study by Lan and Lau [87], in cases considering the combined effect of geometry and vegetation. However, most reported cases showed an increase in temperatures compared to the base scenario, up to +2.1 °C in a case where only geometrical interventions were considered. The increase in temperatures related to geometrical interventions was also reported by Ali-Toudert and Mayer [88] via simulations conducted in ENVI-met for a typical summer day in Ghardaia, Algeria. An urban canyon of height/width ratio of two, and galleries on the ground floor was selected as the base case, which was then compared to an asymmetrical urban canyon due to terraced buildings to one side (the high sky-view factor scenario), and a third scenario with overhanging façades besides the ground floor galleries, present in all three scenarios (the low sky-view factor scenario). All comparisons to the base case resulted in higher MRT values ranging from +0.1 to 1.1 °C under different canyon orientations.

As mentioned before, the general impact on ESTs is higher, which is also true for geometrical changes in façades. Allegrini and Dorer [89] reported temperature reductions, of up to −21 °C, for a simulated scenario of different urban geometries, comparing the maximum wall surface temperature registered for a typical summer day (2 July) in a street canyon with a height/width ratio of two. Nonetheless, these results were particularly high compared to most other scenarios assessed in the study, resulting in an average reduction by 4.08 °C, with the minimum values of 0 °C. These averages are in line with the values reported by Mazzeo and Kontoleon [90], who assessed the peak external and internal surface temperatures of different roof geometries, both lightweight and heavyweight during the summer season in Thessaloniki, Greece and Cosenza, Italy. The results were obtained through a dynamic model, showing a range between −2 and −12 °C for an EST decrease, compared to a flat roof scenario, where the EST fluctuation depended strongly on the orientation of the roof, and thus on the avoidance of direct solar radiation on the surfaces. Finally, Han and Taylor [91] studied the role of retroreflective façades in lowering ESTs in neighboring buildings in an urban canyon placed in Miami. The maximum temperature decreases obtained were between −0.66 and −1.64 °C, with averages ranging from −0.43 to 0.99 °C. Even though the results vary largely in the discussed studies, it seems relevant to point out that no temperature increases were reported when assessing the impact of geometry on ESTs.

Material

Among the reviewed studies, only Yuan and Emura [92] explored the effect of material choices on the EAT, reporting results ranging from −4.91 to +0.94 °C for a ceramic façade with varying albedo values and urban green cover. The study was carried out via simulations in ENVI-Met for 8 November in the context of Osaka Japan. The results showed that solely changing the albedo of the ceramic façade did not help decrease the external air temperature, with a reported temperature differential between −0.07 and +0.94 °C. The same tendency was shown in the study when reporting the resulting MRT, showcasing an increase of the MRT between +0.85 and +1.01 °C when the façade albedo was increased from 0.3 to 0.7. The effect of material changes on the MRT was also explored by Naboni and Milella [27], reporting higher values, up to −4 °C, in temperature decrease though a simulation-based study of the impact of material changes in the MRT during summer days in an urban canyon of W/H = 1 in four locations: Copenhagen, Madrid, Brindisi, and Abu Dhabi. The authors concluded that the window to wall ratio (WWR) appears to be a relevant parameter in the determination of the local MRT, finding a direct relationship between them. Moreover, the use of higher reflectance solutions on façades showed a decrease in the peak MRT, reaching over −4.0 °C during summer in Copenhagen, −3.9 °C in Madrid, −2.5 °C in Brindisi, and −2.0 °C in Abu Dhabi.

The effect of material choices on the EST is reported to be markedly higher as expected, ranging from 0 to −13 °C. Temperature decreases over 10 °C were reported by Doya and Bozonnet [93] and Urrutia del Campo and Grijalba [94]. The former conducted an experimental measurement of façades with a layer of cool paint in a reduced-scale model of a typical urban scene in La Rochelle, France. The application of brown cool selective paint on façades resulted in a maximum temperature decrease between −10 and −13 °C compared to the base case with standard brown coating at times of solar exposure during summer period (14 July to 13 September). Urrutia del Campo and Grijalba [94] reported comparable temperature differentials on the upper threshold of their results, which ranged from −0.3 to −13 °C in their experimental study of the impact of different finishing materials and shading in the ESTs of different façades facing a square in the center of Madrid. The data were gathered via on-site sensors during spring and summer periods; the authors considered granite, red brick, and whitewash as façade finishes under varying solar exposures. The highest reductions were obtained when comparing granite surfaces exposed to the sun to shaded whitewash surfaces (−10.5 to −13 °C), with maximum EST reduction between −7.3 and −9.2 °C when both surfaces received direct solar radiation, and between −0.7 and −3.6 °C when both were shaded, which provides a context to the effect of material choices regarding other environmental variables. The results reported by Wonorahardjo and Sutjahja [95] agree with the previous studies, showcasing EST reductions between −0.2 and −6.6 °C when comparing different façade materials in laboratory conditions. A brick wall, an aluminum composite panel (ACP), a concrete wall, clear glass, and low-E glass were considered in the study, measuring ESTs during the morning for east and west orientations. The minimum reduction (between −0.2 and −0.5 °C) was registered when comparing low-E glass and the ACP, showing similar performance, while the maximum was achieved by comparing low-E glass as the base to concrete (−4.3 to −6.6 °C), agreeing with the conclusions regarding the WWR posed by Naboni and Milella [27] when it comes to the MRT.

Vegetation

As previously mentioned, the incorporation of vegetation on building façades resulted in the highest temperature decrease reported in the reviewed studies on all three indices. Regarding the EAT, Dardir and Berardi [96] and Yuan and Emura [92] reported temperature differentials over −4.0 °C via microclimate simulations for the contexts of Toronto, Canada and Osaka, Japan, respectively. Dardir and Berardi [96] developed and validated a model based on the Urban Weather Generation code to evaluate the impact of urban greenery on the thermal performance of three neighborhoods in the Greater Toronto area during a summer week (27 June–6 July 2019). The model reported reductions on the maximum EAT ranging from −2.0 to −4.6 °C when increasing the vegetation cover on the sites, with average reductions ranging from −1.2 to −3.0 °C. Yuan and Emura [92] studied the effect on the EAT after increasing the urban green cover from 0% to 20% on ceramic façades with different albedos (0.3–0.7). This was conducted via simulations in ENVI-Met, comparing the EAT for 8 November. The temperature differential for the maximum recorded EATs (around 38 °C) ranged between 1.03 and 1.26 °C, while the respective differential for the minimum recorded EATs (around 20 °C) was between −3.88 and −4.91 °C. The ranges mentioned match the experimental results obtained by Lin and Xiao [97] in their assessment of the performance of green façades in the hot humid context of Guangzhou China. A monitoring campaign conducted in different days of August 2017 resulted in maximum temperature differentials ranging from −3.2 to −4.8 °C, showcasing the maximum EAT differentials reported in the reviewed articles. On the other hand, there were simulation-based studies that reported temperature differentials from 0 to −0.7 °C for the contexts of Cairo, Bolzano, and Guangzhou, in Egypt, Italy, and China, respectively [98,99,100,101]. This is regarded as proof that even though there is clear potential for heat mitigation, the incorporation of vegetation should not be taken as a clear-cut measure, but it needs to be carefully assessed considering other design variables and the local context.

The same holds true for the studies that reported on impacts derived from the application of vegetation on the MRT, finding ranges from 0 (no impact whatsoever) to −11 °C. The highest reductions were reported by Yuan and Emura [92] and Lin and Ni [102], through simulations in the contexts of Osaka, Japan and Guangzhou, China. The former reported temperature differentials between −9.65 and −11.64 °C for the maximum recorded MRTs and between −3.95 and −4.73 °C for the minimum ones over a 24 h simulation of ceramic façades with different albedos (0.3–0.7) when increasing the green cover from 0% to 20%. The latter studied the impact of different green façade configurations (on a single side, on both sides, over balconies, in railings, and on a living wall) on a south-facing façade over 10 days in June 2019. The study reported MRT differentials from 0.51 to 6.93 °C [102]. Lastly, minor albeit positive effects were reported by Jänicke and Meier [103] and Djedjig and Bozonnet [104]. The first conducted simulations of vegetated scenarios on a south-facing façade in Berlin, Germany, reporting a decrease in the MRT between −2 and −3 °C. The second reported similar MRT differentials, ranging from 0 to −3 °C from their experimental study involving scaled-down building blocks and urban canyons to assess the urban heat mitigation potential of green walls in La Rochelle, France.

Finally, as expected, the impact of vegetation on the EST was the highest, reaching up to −40 °C, with an average of −6.3 °C EST reduction considering all reviewed studies (which puts the declared value of −40 °C in perspective, identifying it as an outlier). The highest values were reported by Zheng and Keeffe [105] in their simulation-based study of the application of Nature-Based Solutions (NBSs) for cooling in high density residential areas of Shenzhen, China. The assessment considered a brick wall as base scenario, which was completely covered by Ivy and evaluated in different orientations for a day in August 2019. The temperature decrease at the maximum registered ESTs ranged from −6.0 to −41.8 °C, averaging −17.4 °C. Jiang and Zhou [106] reported maximum EST differentials of −13.6 and −15.2 °C for east and west orientations on a summer day through a simulation-based study of an office building with a green façade in Ningbo, China. Görgen and Rossi-Schwarzenbeck [107] comprehensively studied the evaporative cooling potential of vertical greening in three different climate contexts: Madrid, Berlin, and Singapore, for 21 June. The simulation-based study was carried out in ENVI-Met, considering a typical residential building, where vertical greening systems of varying Leaf Area Indices (LAIs = 1, 2, and 5) were applied in either its north or south façade. The highest EST differences on the south façade compared to the bare wall case were found in Berlin (−17.51 to −18.28 °C), with Madrid being the second (−9.88 to −11.0 °C), and Singapore the last (−6.24 to −6.96). For the north-facing façade, all results were contained in the range between −5 and −7.48 °C regardless of the location. This shows a similar effect to vegetation on façades with minimal solar exposure across different climate contexts, but its thermal mitigation potential is expanded for façades directly exposed to solar radiation, due to the added benefit of sun-shading besides the natural evapotranspiration of plants.

Experimental results reported in the reviewed articles are in sync with the data obtained from simulations, with maximum EST reductions of around −15 °C. Malys and Musy [108] experimentally studied the impact of green walls on external temperatures compared to a bare brick wall through prototypes in Geneva, Switzerland during the final week of May 2010. The study reported maximum EST reductions between −9.5 and −14 °C and between −7.8 and −11.8 °C when comparing the bare wall EST against the EST of the substrate and the leaves of the green wall, respectively. These ranges match the results obtained by Fadli and Zaina [109] in their experimental assessment of a novel living wall compared to a bare wall during a summer day in Doha, Qatar. The test setup considered monitoring of the concept in four orientations, reporting the maximum EST differentials of −15.6, −8.4, −8.7, and −10.3 for south-west, south-east, north-west, and north-east orientations, respectively. Bianco and Serra [110] obtained EST differentials between −7 and −8 °C at the moment of maximum recorded ESTs of a bare wall compared to the recorded ESTs of a novel vertical greenery module in an outdoor test cell located in Madrid, Spain. Similarly, Blanco and Schettini [111] reported EST differentials ranging from −5.3 to −7.0 °C in their study of the thermal impact of a green façade over three different days of summer (28 June, 23 July, and 27 August 2016), compared to a south-facing brick façade in Bari, Italy. The results consist of the temperature differentials registered at the maximum ESTs, considering the temperature decrease from green façades composed of two different species: Pandorea jasminoide and Rhyncospermum jasminoide. Lastly, the lowest reduction range for maximum registered ESTs was reported by Jim [112]. The range of −0.7 to −6.0 °C, was obtained from their evaluation of the thermal impact of three different climber plants over a summer day (2 August 2012) in the context of Hong Kong, considering all orientations. The study considered a concrete wall as the base case, obtaining better results on east and west orientations (−2.9 to −6.0 °C) because of the higher solar exposure on a vertical plane, and worst results (−0.7 to 2.0 °C) for the north orientation.

4. Discussion

Table 3. Key lessons from the review on the impact of design choices on the identified environmental problems (E.P.).

E.P.	Material	Geometry	Vegetation
Biodiversity	Reflective materials can create visual disturbances for birds, besides contributing to light pollution, disrupting natural behaviors of nocturnal species [47]. Materials that retain moisture could increase the abundance of species [31]. The use of natural materials, such as wood and earth, besides the increased thermal mass, is preferable to promote the generation of habitats [49].	Reflections from complex geometries could disrupt birds’ and insects’ flight patterns [47]. Self-shading in complex geometries affect plant species differently, limiting sunlight exposure [48].	Dense vegetation can provide habitats, food sources, and shelter for various species. Vegetated façades serve as habitats for spiders and beetles, increasing local biodiversity [31,51,52].
Noise	Absorbing materials in façades are the most effective design measure to increase urban acoustic comfort [57,58,61,62,65,66,71,80]. Absorbing panels in balcony ceilings have been reported as a useful integral measure to reduce noise at the pedestrian level [57,63]. Besides absorptivity, the diffusivity of façade cladding was reported to be relevant to mitigate noise levels in courtyards [82].	Geometries influence the propagation and reflection of sound waves. Inclined planes and triangular shapes help reflecting sound waves to the exit of urban canyons [77]. Louvres, balconies, and eaves with high repetition in façades help deflecting or absorbing sound waves [60,61,72,75,77].	A controlled use of vegetation increases sound absorption in urban canyons, reducing reflections and reverberation, especially at high frequencies [64]. The shielding effect is related to the depth and density of the vegetated layer and the composition and depth of its substrate [83]. Plants with large leaves rooted in soil substrate exhibit an increased absorption coefficient and a higher scattering coefficient at low and high frequencies, respectively [78]. Absorption decreases with a higher moisture content [78].
Urban Heat	Surface finishes decrease external surface temperatures (ESTs) and mean radiant temperatures (MRTs). But their impact on the air temperature (EAT) is minimum, considering a higher albedo [92,94] or the use of cool paints [93]. The use of highly reflective materials in façades show a decrease in the peak MRT [27]. An increase in the window-to-wall ratio results in a higher EST and MRT [27,95].	The role of geometry is bigger at the urban scale, modifying canyons proportions to lower the sky-view factor, increasing shade [89]. Complex geometries that reflect sunlight upward have a limited effect in decreasing the EST [91]. This effect is reportedly higher in tilted roof geometries, compared to flat roofs [90]. Geometries may be used to enhance wind flows near façades, but impact on the EAT is limited [87].	The incorporation of vegetation in building façades is the most effective measure to decrease urban temperatures, reaching noticeable reductions in the EAT [92,96], although there are cases where its impact is negligible [98,99,100,101]. Increasing the Leaf Area Index (LAI) improves the impact of vegetation, but even low values may have a noticeable impact depending on the context [105,106,107].
Air pollution	There is not enough information in the surveyed sample.	Building geometries can condition wind behavior in dense urban contexts, indirectly affecting the dispersion of pollutants [40]. Geometry induced surface temperatures can support easier dispersion of contaminants in an urban canyon if the windward wall is warmer than the leeward side [33,41]. The shape and position of balconies and protrusions influence the airflow near façades, affecting washout rates. Separated balconies are preferred compared to continuous horizontal balconies for air mass transfer [42].	A green wall in an urban canyon could lead to reductions of up to 35% and 50% in the concentrations of NO2 and PM10, respectively, with higher effectivities found in species with high Leaf Area Indices (LAIs) [44,45]. The pollutant-absorbing effectiveness of a green wall does not seem to change along its height [46].

shows a summary of key lessons from the reviewed studies regarding the impact of the identified main design variables (material, geometry, and vegetation on the defined urban environmental issues (air pollution, urban heat, urban noise, and biodiversity in cities).

Overall, the review showed that, as expected, the application of vegetation carries relevant benefits on all the defined environmental issues, cementing the importance of fostering more green areas in urban centers, for healthier cities for all species in the urban ecosystem. Besides the evident role of vegetation in the promotion of biodiversity, the review showed that the use of natural materials in building envelopes, such as wood and earth, are preferred for the generation of habitats, although this must follow a careful design to avoid risking the integrity of façade elements, compromising their capacity to respond to indoor requirements. Moreover, reflections from shiny building surfaces were singled out as problematic for birds and insects, while the incorporation of protrusions or complex geometries in façades could be helpful for increasing moisture retention in specific areas by modulating solar irradiance levels in exterior surfaces. Hence, an opportunity is detected regarding the exploration of biodiversity driven geometrical parameters for façade design.

Regarding urban noise, there is wide agreement on the high effectivity of absorbing materials in façades to improve urban acoustic comfort; however, these porous materials tend to be less durable than hard surfaces, so their application needs to be carefully considered under an integral design approach. Moreover, the presence of protrusions in façades was declared as beneficial in deflecting and absorbing noise alongside urban canyons, especially if their geometry helps reflecting noise outside of it, while the positive effect of vegetation generally considers noise reflection and scattering by leaves, and the absorption provided by the substrate, with higher depth being particularly effective for low-frequency noise.

When it comes to the mitigation of urban heat, the use of vegetation in building façades is by far the most effective strategy reported, by taking advantage of the natural evapotranspiration effect of plants, besides the shading that they provide to the building surface underneath. Besides vegetation, the application of reflective surfaces with high albedos is regarded as beneficial, preventing heat absorption in buildings. However, the direction of the reflected solar radiation should be carefully considered since it could end up heating up the pavement or surrounding buildings. Hence, tilted planes or façade geometries that reflect solar radiation upwards would be preferred.

Finally, regarding air pollution abatement, vegetation is again the most effective strategy, particularly when considering species with a high Leaf Area Index (LAI). About material choices, even though there are specialized coatings for building surfaces available in the market, which absorb and filter Nitrous Oxides through photocatalysis (Titanium Dioxide-based coatings), the review did not result in relevant information regarding the base materials being used in façades and their impact on air pollution. Hence, more studies are needed to understand and quantify if façade material choices have a discernible effect on outdoor air quality. In terms of geometrical design choices, it is mentioned that horizontal elements in the façade could affect air flows near buildings, which in turn diminishes the dispersion of pollutants. In this sense, it is preferred to consider discontinuous protrusions instead of continuous horizontal balconies, for instance, to allow air currents near building surfaces. It is relevant to mention that building geometries that not just allow, but also boost air currents near façades could both improve the dispersion of pollutants and help lowering surface temperatures, showing the potential in synergic design strategies that tackle different issues at once.

To expand on this synergic design potential, a more detailed assessment of the role of the selected design variables on the multi-domain environmental performance of façades was conducted. Thus, specific properties related to each design variable were identified, based on their explicit mention in the reviewed articles, categorizing them in terms of their appropriateness to respond to each environmental issue.

Table 4. Identified properties related to the design variables and their desirability in tackling the environmental issues. Darker colors reflect positive attributes for each environmental issue.

Design Variable	Properties	Air Pollution	Biodiversity	Noise	Urban Heat
Material	Reflectivity	-	No	Yes, but depends on direction	Yes (directed upwards)
	Absorptivity	-	Yes	Yes	No
Geometry	Inclination/tilt	-	-	Yes	Yes
	Protrusions	Only if not continuous	Yes, but beware of excessive self-shading	Yes	Yes, ideally not continuous
Vegetation	Depth	Yes	Yes	Yes	-
	Leaf Area Index	Yes	Yes	Yes	Yes
	Moisture retention	-	Yes	No	Yes

shows the identified properties and their reported role in each environmental domain, detailing if they generate a positive effect (yes), a negative effect (no), or no discernible effect (-) based on the reviewed sample. It is important to point out that this table does not seek to be exhaustive but merely presents relevant properties extracted from the review, seeking to visualize compatibilities or incompatibilities between multi-domain requirements.

The relevant material properties identified consist of reflectivity and absorptivity. In general terms, noise and urban heat could benefit from reflective surfaces, if properly oriented. However, it was strongly reported that these could generate problems for local birds and insects. On the other hand, absorptive surfaces could prove beneficial for noise reduction, habitat generation, and moisture retention; however, absorptive surfaces could end up storing unwanted heat during the day to be released over nighttime, increasing the urban heat island effect of a specific area.

Regarding façade geometry, an architectural potential is identified in the use of carefully designed protrusions, given their positive role in contributing to the mitigation of several issues, if they are not continuous, to avoid affecting air currents near façades. The use of tilted surfaces could potentially increase their effectivity for mitigating noise and heat; however, it would be important to avoid strong light reflections by carefully choosing the surface material.

Finally, as stated, vegetation not only appears to be the most beneficial design strategy to be considered, but these benefits also extend to the Leaf Area Index (LAI), increasing the effectiveness of vertical greenery in tackling every environmental issue defined in the study by favoring species with a higher LAI. The depth of the green wall also plays a relevant role, but more information is needed to assess potential impacts regarding urban heat in specific contexts, because even though a deeper substrate layer could mean higher moisture retention, it could also increase heat absorption. Similarly, high moisture retention has been found to be detrimental to noise mitigation, diminishing the absorptivity of the exposed surface. Hence, it is fundamental to equalize material and geometrical properties and calibrate the design of the building envelope seeking appropriate responses to well-defined urban environmental objectives.

In that regard, the identification of relevant façade properties in terms of their reported response to distinct and sometimes clashing urban environmental issues is presented as the main novelty from the systematic review. Most studies focus exclusively on one environmental issue to add new detailed knowledge on each domain. However, it is important to consider the combined and even counterproductive effect of design decisions on different urban environmental issues as a first step to derive clear guidelines or tools that support environmentally responsible design based on particularities of local urban contexts, which helps design teams to visualize potential issues and consciously prioritize building design choices to improve the livability of urban centers.

5. Conclusions

This paper aimed at systematically outlining and discussing the impact of façade design choices on four defined urban environmental issues: air pollution, biodiversity loss, noise pollution, and urban heat. It was conducted through a systematic review of scientific studies following the PRISMA protocol, gathering a database of 121 articles, which were then explored to answer two guiding questions: (i) What research is being conducted regarding the impact of the design of building envelopes on urban environmental problems? and (ii) What impacts on the urban environment have been reported regarding different façade design aspects?

Answering the first question, most of the articles tackle urban heat, followed by the impact of façades on external acoustic comfort, being fields of increasing interest in recent years. Articles delving into the impact of façade design on urban biodiversity and air pollution abatement had the least number of matches, which shows a clear knowledge gap that needs to be further researched, especially regarding the potential effect of material and geometrical design choices.

To answer the second question, three types of façade design aspects were defined: material, geometry, and the use of vegetation, aiming at identifying and discussing explicit impact data from the gathered studies. The inclusion of vegetation in building envelopes was reported to generate the highest benefits in all the environmental issues considered in the review. Moreover, it was found that higher Leaf Area Indices result in better performances in all cases, showing the effectiveness of richly vegetated building surfaces. Nonetheless, even though more vegetation in cities is unarguably desirable, it usually comes with extra maintenance costs and water requirements, which could be difficult to meet in certain climatic and economic contexts. Thus, it is important to look for the effect of other design strategies as well.

About geometrical design choices, the incorporation of protrusions in façades was found to present interesting opportunities, carrying interesting benefits for all environmental issues if designed correctly, with particular relevance given to their non-continuous arrangement to avoid interfering with air currents near the façade for pollutant dispersion and heat mitigation purposes. Finally, about material design choices, the main contested properties are absorptivity and reflectivity in façade finishes. On the one hand, absorptive surfaces are the most effective strategy for urban noise abatement and could potentially support biodiversity by allowing higher moisture retention with porous materials. However, heat absorbing surfaces could end up rising nighttime temperatures, increasing the local urban heat island effect, besides being less resilient compared to hard surfaces. On the other hand, upward-facing reflective surfaces are effective for solar heat dissipation and for reducing noise pollution at the pedestrian level, although their design would need to account for potential discomfort created for upper floors in mid- and high-rises. Notwithstanding the mentioned benefits, reflective surfaces have been found to be dangerous to birds and insects, disrupting their flight patterns and provoking injuries and fatalities by building collisions. Thus, these and other potential incompatibilities need to be carefully assessed, to adequately equalize the application of diverse materials and geometries in façades according to specific local requirements, contexts, and urban forms.

Finally, it seems important to reiterate that the work presented in this article does not seek to be exhaustive but presents a panorama of fields of study that are gaining increasing attention. Likewise, the identification and discussion of impact data are limited by the searched databases and studies selected, focusing on the external performance of the defined façade design aspects. Therefore, more studies are needed in less explored scientific fields but even more so, on cross-effects and the multivariant impact of the design of the building envelope on various socio-environmental domains affecting both the quality of the interior space and the local urban context of our buildings under a holistic framework.

In this regard, this systematic review seeks to orient future work on these topics with the goal of supporting the development of new façade concepts and components guiding conscious design decisions that seek to improve the urban ecosystem under a regenerative paradigm for architectural interventions in our cities.