Enhancing Outdoor Environmental Comfort: A Review of Façade-Surface Strategies and Microclimate Impacts

Abstract

Building façades traditionally focus on enhancing indoor environmental quality and improving energy performance, but undermine their influence on Outdoor Environmental Comfort (OEC), including thermal, acoustic, and visual conditions. With technological advancements in envelope design, research on new materials and green systems has been introduced in the last few decades. This review examines the role of two key elements—façade materials and green façades—in shaping OEC. A total of 41 peer-reviewed studies (24 on urban scale and 17 on building scale) were categorized into three focus areas: (1) outdoor thermal comfort; (2) outdoor acoustic comfort; and (3) outdoor visual comfort. The analysis was structured across three levels: (a) Performance Determinants; (b) Metrics/Models; and (c) Material or Façade Types. We proposed this analytical structure to highlight the interactions between building façades and OEC domains (thermal, acoustic, and visual comfort). Our results showed façade treatment can impact all three comfort factors related to OEC, but trade-offs must be evaluated. Moreover, the findings highlighted that additional research is required to cover variations in both climate and context conditions, due to their close association with the OEC. Finally, the conceptual framework is presented to synthesize the three comfort domains for sustainable outdoor environments.

1. Introduction

Climate change and global warming are among the most pressing global challenges of the coming decades. [1]. Buildings significantly contribute to these issues through their energy consumption, emissions, and material choices. At the same time, global population growth is projected to reach 10.4 billion by 2100 [2], accompanied by intensified urbanization. As cities densify, the built environment increasingly alters Outdoor Environmental Comfort (OEC) as defined by microclimates, ambient noise, and visual glare [3]. These directly impact the outdoor comfort levels of people using outdoor urban spaces, which are key to the success of social life [4]. Understanding and improving outdoor comfort is therefore essential to achieving healthier, more sustainable urban communities.

OEC plays a vital role in shaping urban life, influencing how often and how comfortably people use outdoor spaces. It is determined by a combination of climatic conditions, urban morphology, and building elements—especially façades. Traditionally, façades have been designed to enhance indoor environmental quality and energy efficiency [5]. However, as the impacts of the Urban Heat Island (UHI) effect become more severe, its role in influencing outdoor comfort is gaining attention. While horizontal surfaces are often implicated in UHI, vertical façades also contribute meaningfully by altering heat retention, airflow, and solar exposure. Façade properties—such as reflectivity, thermal mass, and material texture—can significantly affect Outdoor Thermal Comfort (OTC) [6,7]. Similarly, façades influence Outdoor Acoustic Comfort (OAC) by amplifying or diffusing sound through their geometry and absorptive properties [8]. In terms of Outdoor Visual Comfort (OVC), material reflectance and surface color contribute to glare intensity and overall luminance conditions [9]. Taken together, the façade is not just a barrier but a key interface for shaping Outdoor Environmental Quality (OEQ).

In a façade assembly, the exterior surface treatment plays a key role in defining the OEQ. This treatment can be seen through the lens of façade materials and green façades [10]. Properties of façade materials, such as reflectivity, can impact outdoor OEC by contributing to UHI. Studies report that replacing conventional masonry with high-albedo or retroreflective finishes lowers façade-surface temperatures by roughly 10–14 °C and improves pedestrian thermal-stress indices by about 3 °C in hot-dry and humid-subtropical settings [11,12]. Similarly, studies on green façades showed that cooling through evapotranspiration and shading could drop the façade surface temperature by 2–4 °C, and acoustic absorption reduced the noise level by 40 dB [13,14,15]. Hence, both these categories encompass a variety of façade treatments that impact OEC.

Façade treatments span traditional masonry and advanced composites engineered for thermal, acoustic, and visual performance [5,6] In parallel, green façade strategies such as living walls and vertical greenery systems are increasingly used to improve outdoor comfort [16]. As we race against the continuous development of these design strategies in building envelope design, there is a need to review the advanced materials and vegetation strategies used in building façades. This review focuses on both material-based and vegetative façade interventions, highlighting their mechanisms and impacts on outdoor comfort.

Building façades are an important element of any urban fabric [17]. The material treatment of these façades can alter the OEQ near the immediate exterior zone of buildings to the public exterior realm of cities, such as urban canyons and plazas. Studies in topics of building façades and OEC have focused either on building scale or urban scale [9,18,19]. In either case, the impact is significant at a human comfort level and must be taken into consideration in the study. This study aims to identify the current state of the art in this topic.

Moreover, although numerous studies address individual aspects of outdoor comfort, few have synthesized the influence of façade design across multiple domains of OEC, namely OTC, OAC, and OVC. This review addresses that gap by providing an integrated analysis of how façade materials and green systems influence thermal, noise, and visual aspects of outdoor comfort levels. To support holistic decision-making in design, a conceptual framework is proposed to illustrate cross-domain interactions and trade-offs.

2. Background

To gain a comprehensive understanding of OEC, it is essential to understand each domain’s technical definition and related metrics or indices. Below is a synopsis of each domain.

2.1. Outdoor Thermal Comfort

Thermal comfort refers to the human perception of ambient environmental conditions. According to ASHRAE, it is defined as “the condition of mind which expresses satisfaction with the surrounding thermal environment and is assessed by subjective evaluation” [20]. While it was initially studied in the context of indoor environments, thermal comfort in outdoor settings has gained increasing attention due to the compounded effects of climate change, urbanization, and the UHI phenomenon. Building façades play a significant role in influencing OTC by modifying microclimatic conditions and shaping the usability of outdoor spaces.

This section reviews twenty-three studies that examine the impact of façade materials on OTC. Similar to indoor thermal comfort research, OTC is influenced by climatic variables (air temperature, humidity, wind velocity, and mean radiant temperature), as well as physiological and behavioral factors. Over a hundred indices for assessing OTC have been proposed [21,22], of which four are widely used: Physiological Equivalent Temperature (PET), Universal Thermal Climate Index (UTCI), Predicted Mean Vote/Percentage People Dissatisfied (PMV/PPD), and Standard Effective Temperature (SET*)

• Physiological Equivalent Temperature (PET) is based on the MEMI heat budget model using variables metabolic activity (M), physical work output (W), net radiation of the body (R), convective heat flow (C), latent heat used to evaporate water that diffuses as vapor through the skin (E_D), sum of heat for air heating and humidification (E_Re), heat loss via sweat evaporation (E_Sw), and body heat storage for heating or cooling (S) [23]. Its equation is written as:

  M + W + R + C + E_D + E_Re + E_Sw + S = 0
• Universal Thermal Climate Index (UTCI) is derived by coupling the thermo-physiological UTCI-Fiala multi-node model [24] and the clothing model [25] for a given meteorological condition. Key variables in its calculation include air temperature (T_a), mean radiant temperature (T_r), water vapor pressure (p_a), wind speed (v_a), and deviation from the air temperature (Offset). The equation is written as below [26]:

  UTCI (T_a, T_r, v_a, p_a) = T_a + Offset (T_a, T_r, v_a, p_a)

The heat-balance equation for the UTCI-Fiala model is based on the energy budget from: (1) overall body heat exchange through metabolic rate activity (M), mechanical power (W), and change in heat content of the body-storage (S); (2) peripheral (skin) heat exchanges through turbulent flux of sensible heat (Q_H), radiation budget (Q*), and turbulent flux of latent (Q_L); (3) heat (passive diffusion of water vapor through the skin) through turbulent flux of latent heat (sweat evaporation) (Q_SW); (4) respiratory heat exchanges through respiratory heat flux (sensible and latent) (Q_Re); and (5) thermal environmental parameters, namely, air temperature (T_a), mean temperature (T_mrt), air speed relative to the body (v), and partial vapor pressure (e). Its equation is written as [27].

M − W − [Q_H (T_a, v) + Q* (T_mrt)] − [ Q_L (e, v) + Q_SW (e, v)] − Q_Re (T_a, v) ± S = 0

3.

Predicted Mean Vote/Percentage People Dissatisfied (PMV/PPD) was derived by O. P. Fanger [28] using variables, namely metabolic rate (M), external work (W), and total heat loss from the body (H_loss). The PMV equation in its simple form is given as

PMV = [0.303 × e^−0.036M + 0.028] × [(M − W) − H_loss]

Heat_loss is computed from convective heat loss (Qc), radiative heat loss (Qr), evaporative heat loss from skin (Qe), and heat loss through respiration (Qres), and is written as

H_loss = Qc + Qr + Qe + Qres

4.

Standard Effective Temperature (SET*) was derived in Berkeley by [29] and is computed using two-node thermoregulation models and heat-balance equations

2.2. Outdoor Acoustic Comfort

Acoustic comfort refers to the perceived well-being and satisfaction with the acoustical conditions of a given environment [30]. It is a critical factor in both indoor and outdoor settings, influencing how individuals experience and interact with their surroundings. In indoor environments, acoustic comfort pertains to a building’s ability to minimize external noise and create a quiet, distraction-free space that supports productivity and social interaction [31]. Similarly, in outdoor environments, acoustic comfort plays a crucial role in shaping the urban experience, significantly affecting the public’s enjoyment and overall perception of urban spaces [32]. A key element influencing acoustic comfort is the building envelope, which acts as a barrier against noise and contributes to creating acoustically balanced spaces. Numerous studies have highlighted the significant impact of building façades on occupants’ acoustic experiences [33]. In urban settings, façades can enhance outdoor acoustic comfort by mitigating unwanted noise, and their effects can be classified into three main categories: sound reflection, sound absorption, and sound production [34]. Some of the common metrics used for the acoustic performance of building façades include Sound Transmission Loss (STL), Sound-Pressure Level (SPL), Sound-Absorption Coefficient (α), and Noise-Reduction Coefficient (NRC). Calculations of each of these metrics are given below:

• The Sound Transmission Loss (STL) could be calculated using the equation below:

  TL (dB) = 10 × log (W1/W2)

  where ‘W1’ is the sound power incident on a wall, and ‘W2’ is the sound power transmitted through a wall
• The Sound-Pressure Level (SPL) equation could be written as

  SPL (dB) = 20 × log10 (p/p0)

  where ‘p’ is the measured sound pressure and ‘p0’ is the reference sound pressure.
• The Sound-Absorption Coefficient (α) is calculated as

  α = l_a/l_i

  where ‘l_a‘ = the sound intensity absorbed by the material and ‘l_i’ = the incident sound intensity.

These metrics offer valuable insights into how various façade materials and configurations impact sound propagation and overall acoustic conditions. This section reviews the impact of façade materials and green walls on outdoor acoustic comfort. The findings offer a deeper understanding of how design interventions, such as material selection and vegetation integration, can mitigate noise and enhance urban soundscapes.

2.3. Outdoor Visual Comfort

Visual comfort refers to lighting conditions that prevent strain, discomfort, or distractions. The EN standard defines it as “the subjective state of visual health caused by the visual environment.” [35]. Although horizontal illuminance remains the default design metric, field studies show that vertical eye illuminance and luminance-based indices, such as luminance ratios, Daylight Glare Probability (DGP), and Daylight Glare Index (DGI), correlate more strongly with perceived comfort. Preferred work-plane levels typically range from 100 to 3000 lux, with most occupants favoring levels between 300 and 2000 lux. Because no single index performs reliably across all daylight and electric-lighting conditions, DGI remains the most widely used practical benchmark, underscoring the need for a more transparent framework to guide metric selection and to develop more comprehensive visual-comfort indices [36,37].

While commonly studied indoors, visual comfort is equally important in outdoor environments. OVC results from the interaction between materials in the outdoor environment and lighting levels. Although both horizontal and vertical surfaces contribute to the visual comfort, this review focuses mainly on the latter—building façades. The focus of visual comfort studies has been either to mitigate the visual glare or to enhance visual pleasure. A comprehensive understanding of visual comfort in outdoor environments is best achieved through an examination of the state of the art in factors and performance indicators.

3. Methodology

This study is a comprehensive review that synthesizes findings from 41 peer-reviewed publications, including journal articles, conference proceedings, and review papers, to evaluate how building façades—particularly materials and vegetative systems—influence OEC. The review emphasized studies that addressed performance metrics, contextual variables (e.g., climate, orientation), and façade mechanisms.

The literature search was conducted using three major academic databases—Scopus, ScienceDirect, and Google Scholar—chosen for their comprehensive coverage of high-impact studies in architecture, urban design, and environmental science. The keyword selection was based on two sets of queries: first, for thermal, acoustic, and visual comfort; and second, for materials and green façades. The following Boolean operators were used to identify the scope of work in this study:

• (“outdoor thermal comfort” OR “exterior thermal comfort” OR “external thermal comfort” OR “ambient thermal comfort”) AND (“building façade” OR “building envelope” OR “exterior wall”)
• (“outdoor visual comfort” OR “urban visual comfort” OR “visual comfort”) AND (“building façade” OR “building envelope” OR “exterior wall”)
• (“urban acoustic” OR “acoustic comfort” OR “outdoor noise”) AND (“building façade” OR “building envelope” OR “exterior wall”)
• (“vertical green systems” OR “green walls” OR “living walls” OR “material performance”) AND (“building façade” OR “building envelope” OR “exterior wall”).

The reviewed articles were selected from works published between 2010 and 2024, with a specific focus on urban outdoor environments and façade strategies that impact at least one domain of OEC. The selection process is illustrated in Figure 1, following a PRISMA-style flow diagram for transparency. The identification of records from the databases included 920 articles. All duplicated articles were removed before screening. In the screening process, titles and abstracts were screened to eliminate unrelated records. Next, all the full papers were screened to ensure the following exclusions were met: (1) focusing on indoor environments; (2) lacking a direct focus on façade materials or green façades; and (3) any non-English-language and non-peer-reviewed publications. Finally, a total of 41 articles meeting these criteria were included in the review. Refer to Figure 1 for the flow diagram on the selection of works.

After screening and eligibility checks, the selected studies were grouped into three comfort categories: OTC, OAC, and OVC. Within each category, studies were analyzed to assess the role of façade materials and green wall systems in influencing environmental parameters, including temperature, noise levels, and visual glare. Furthermore, the studies were categorized into two levels: building and urban. The building category considers studies near buildings, while the urban category encompasses buildings in urban blocks or neighborhoods. This review was guided by the following research questions (RQs):

• RQ1: What types of materials and vegetation have been investigated in façade design for improving outdoor comfort?
• RQ2: Which performance metrics are most commonly used to assess their impact on OTC, OAC, and OVC?
• RQ3: What measurable outcomes have been reported, and how can these findings inform the design of future climate-responsive façades?

This methodology provides a transparent and structured foundation for synthesizing interdisciplinary research on façade-driven outdoor environmental comfort in urban contexts.

4. Results

Based on 41 articles, a literature review was conducted by grouping the total number of articles into the three OEC domains: OTC, OAC, and OVC. Next, the façade categories, façade materials, and green façades were extracted to create a subgroup from which their types were extracted for each domain. The common OEC metrics used in these studies were then gathered along with their findings. Additionally, studies were then grouped under buildings or urban scale, as another layer of information. All these groupings are synthesized and highlighted in

Table 1. Synthesis of review findings.

References	Category	Material/Green Façade Types	Building/Urban Scale	Domain (No. of Studies)	Common Metrics	Summary of Findings
[6,7,10,11,12,17,38,39,40,41,42,43,44,45,46]	Material	Brick, Stone, Concrete, RR, DHR, RR, BIPV, HPL, Ceramic, Aluminum, Glass	Building (9)/Urban (14)	OTC (23)	PMV, MRT, PET, UTCI, surface T, WBGT, SET*	OTC is influenced by material properties that have a strong relationship with climate and design parameters. Materials with high reflectivity, light colors, and low thermal mass can help lower surface and air temperatures, thereby reducing the intensity of urban heat islands (UHI). Similarly, green façades can decrease surface temperatures and contribute to lower average physiological equivalent temperature (PET) levels.
[10,13,14,16,18,47,48,49,50]	Green Facade	Living Wall System, Green walls with climbing plants
[8,19,51,52,53,54,55,56]	Material	Concrete, Brick, Cork Wood, Fiber-cement, Glass, HPL	Building (6)/Urban (7)	OAC (13)	SPL, LAeq, EDT, STI, NR, RT20, D50, Sound Absorption Coefficient	OAC is influenced more by design parameters than by material properties. The geometry of façades and the sound absorption coefficients of materials have a significant impact on urban noise propagation. Green wall systems not only reduce noise but also serve as effective, dual-function interventions that enhance both aesthetics and comfort.
[15,57,58,59,60]	Green Facade	Living Wall System, Green walls with climbing plants
[9,61,62,63,64]	Material	N/A	Building (2)/Urban (3)	OVC (5)	DGP, DGI, DG, E_cyl, E_h, E_v	OVC is an under-researched area that closely relates to material properties and design parameters. Certain materials with high specular reflectivity and saturated finishes contribute to glare, visual fatigue, and pupil dilation, which can be mitigated through appropriate design.

.

The results were based on an analysis structured and conducted at three levels for each of the two categories: materials and green façade, for each of the three comfort factors. The first level involved investigating the “Performance Determinants” through the lens of Material/System Properties, Design Parameters, and Climatic Context, which contributed to OEC. The second involved extracting the popular “Models/Metrics” used for each category. The third level identified the “Material Types” or “Green Façade Types” in the literature. A detailed table of this analysis is found in the Supplementary Materials listing the findings from each article.

4.1. Outdoor Thermal Comfort and Façade Materials

4.1.1. Performance Determinants

The review results highlight the primary and secondary factors that exhibit the relationship between materials in the envelope system and OEC. Primary factors include material properties, while secondary factors include urban design parameters and climatic context.

Material properties as primary factors are direct factors impacting the outdoor environment and include: solar reflection (SR) & emissivity (e) [10,11]; thermal mass (T_m) [10]; heat retention (H_r) [10,38]; thermal flux (T_f) [38]; albedo (A) [7,11,39]; solar absorption (SA) [40]; retro reflectivity, where light returns to its source [6,11]; diffused reflectivity, where light scatters at different angles [11]; solar absorbance in Building Integrated Photovoltaic (BIPV) [41]; and color & texture [10,42].

Climatic context and urban design parameters are the secondary factors that indirectly impact the material performance of an envelope system. Climatic contexts include the Tropical rainforest (Af) context of Singapore [41]; the Hot semi-arid (BSh) context of Iraq [17], Agadir, Morocco [43], Shiraz, Iran [10], Biskara city, Algeria [38] and Errachidia, Morocco [43]; the Humid subtropical (Cfa) context of Toyohashi, Japan [11] and Parma, Italy [42]; the Temperate oceanic (Cfb) context of London, UK [39] and Zurich, Switzerland [41]; the Hot summer Mediterranean (Csa) context of Fez, Morocco [43], and Rome, Italy [6]; the Humid continental (Dfb) context of Munich, Germany [40]; and the Humid Continental (Dwa) context of Tianjin, China. Urban design parameters include sky view factor (SVF) [42]; aspect ratios [11,38,43]; orientation [38]; canyon width [41]; and urban geometry and building height [17,41,45].

Urban parameters like aspect ratios, canyon widths, and orientation are essential to UHI, and their connection with façade materials has been extensively studied to understand the OTC levels in these areas. Materials show high heat retention or cooling capacity in specific urban configurations. For instance, while brick can cause a heat retention range varying from 70% to 93% within the same canyons, limestone, in a north–south orientation, can provide a cooling efficiency range from 85% to 100% in deep and shallow canyons [38]. Another study showed that retroreflective (RR) walls demonstrated better performance in higher urban aspect ratios [11]. Other urban parameters include ground surface material, building typology, building height, and canyon width. These parameters were explored in a study finding that ground surface material was the most influential factor, and street width was the most effective factor affecting the OTC levels [41]. Another study showed that façades in high-rise configurations have a higher impact on OTC levels than in low-rise urban settings [17]. Urban geometry also affects solar availability on building façades, impacting the OTC levels of an outdoor urban space. A recent study on 24 urban configurations in London explored the amount of solar availability on façades, highlighting that three urban parameters, namely layout complexity, standard deviation of building height, and directionality, affected the diffuse and direct solar irradiance, thus impacting the OTC levels [45]. A study in Moroccan cities exploring the heat flux from façades with conventional brick walls and historic thick walls showed that medium aspect ratios are best suited to achieve acceptable PMV levels [43].

Physical properties, such as color, texture, emissivity, and solar reflectance, have been crucial in understanding the relationship between building façades and OTC levels. A study examining how physical traits affect PET levels found that materials with high albedo (light-colored cement, travertine, or aluminum) raise the mean radiant temperature and PET index despite having lower surface temperatures [10]. However, a comparative study of two different reflective materials (with high albedo), RR and diffuse highly reflective (DHR) materials with similar solar reflectance showed that, unlike DHR, RR exhibited better performance in reducing thermal environment indicators, namely (Wet-Bulb Globe Temperature (WBGT) & new Standard Effective Temperature SET*); and radiation indicators (sol-air temperature (SAT) & change in operative temperature (COT) [11]. A similar experimental study investigating the albedo values of RR material on façades showed that albedo increased from 38.2% to 42.3%, with the incident angle of the light beam ranging from 8° to 60° angles [6]. It is also found that materials with high thermal mass and low emissivity can result in lower air temperatures [10]. A neighborhood-scale computational study in the humid continental climate of China, exploring the effect of wall albedo change from 0.3 to 0.7, showed a decrease in the wall surface T [7].

4.1.2. Models/Metrics

The review also highlights various performance indicators used to understand the impact of envelope materials on OTC. Two performance indicator categories were used in these studies: (a) OTC metrics such as PMV [17,43], UTCI [40,41], SET* & COT [11]; PET [10,38]; and (b) meteorological variables include surface temperature (surface T) [7,10], air temperature (air T) [41,42], direct and diffuse solar radiation [45]; mean radiant temperature (MRT) [10]; wet-bulb globe temperature (WBGT), and sol-air temperature (SAT) [11].

4.1.3. Material Types

The review identified two main categories for material types that were studied to evaluate their performance. These are conventional and advanced materials.

Conventional materials: Most research focused on the first category—conventional materials—that included materials such as brick [10,17,38,39,40,42,43], which was found to be the most widely researched material. This was followed by concrete [10,17,38,40,42]. Other conventional materials included aluminum [10], glass [39], cement [10,40], and different types of stones [10,17,38,42].

Studies have shown that certain materials have a significant impact on OTC levels, particularly in a specific climatic context. A comparative study of twenty commonly used façade materials in sunny and shade conditions in the hot-arid climate of Iran. It showed that façade surface temperature (T_s) is affected by solar reflection, emissivity, and heat capacity properties of materials [10]. The studied materials were brick (cream/red), cement (white), travertine (white/cream/brown/gray), granite (black/gray), ceramic (brown/cream), porcelain (cream/gray), artificial stone (beige), marble (cream/white), aluminum composite (gray), concrete (multicolor), and living wall (continuous/ modular).

Another study in the hot semi-arid climate of Iraq explored five façade materials, namely clay bricks, thermo-stone, glazed panels, granite finish, and concrete blocks, to assess their impact on OTC levels measured through PMV [17]. The findings showed that burnt brick had the worst impact, while thermos-stone and granite walls had the best effect on PMV levels. A study in the hot-dry climate of Biskara, Algeria, investigated four façade materials—hollow brick, concrete hollow block, [adobe] mud brick, and limestone—deriving six material thermal flux categories and highlighting the best material in each case [38]. For instance, limestone was the best material, with a high cooling category (above 60–100%) while brick, concrete, and adobe fell into the extreme heat category.

A large-scale physical model study of a neighborhood in London, comparing the albedo for brick wall, aluminum cladding, and curtain wall, highlighted that the curtain wall, as compared to the aluminum and brickwork façade material, exhibited a lower contribution to the canyon albedo during the day [39], thus implying its poor contribution to acceptable OTC levels in summer. A simulation study of three façade materials, brick, concrete, and plastered insulation with two colors, and their impact on UTCI, conducted in the continental climate of Munich, showed that the concrete material with high solar absorption (80%) had the least UTCI stress, while plastered insulation with low solar absorption (30%) had the highest UTCI stress [40].

While most of these studies showed a strong relationship between materials and OTC, a study on conventional materials stoneware panels (light gray and red colors), high-pressure laminate (HPL) panels (light gray and intense red colors), exposed brick, and concrete in marine climate of Italy showed significantly fewer differences in the simulation results of air T for the case study [42]. The authors highlight that the sky view factor (SVF) of the urban canyon (nearly 1) could have played an important role in these findings.

Advanced materials: Besides conventional materials, there has been an increased interest in advanced materials such as RR [6,11]; DHR [11]; and BIPV [41]. It has been found that reflective materials/coatings in façades help mitigate the UHI effect in urban canyons. There have been studies that explored reflective materials, namely RR material—that returns the light back to the source material—and DHR material—that reflects light at many angles (i.e., scatter light), rather than just one. An experimental study using RR and DHR wall models showed that RR walls showed improved OTC levels (SET* and COT as thermal comfort indices) compared to DHR [11]. However, the Computational Fluid Dynamics (CFD) analysis of the same wall models in this study showed a negligible effect on air temperature.

While high solar reflectance and high solar emittance are ideal material characteristics to reduce the UHI effect, there is a constant rise in the use of BIPV façades to help with onsite energy generation. The inherent solar radiation-absorbing characteristic of BIPV makes it a heat source and thus amplifies the UHI effect when it is used as a façade material. However, their relatively low thermal mass in assembly design helps with quick heating and cooling of the rear-ventilated surfaces. A study on BIPV façades in eighty different urban simulation scenarios showed that the use of BIPV in different climates produces different OTC level ranges. Lower air T ranges result in narrower UTCI ranges, as seen in Singapore, while the opposite is true in Zurich, which has higher ranges [41].

4.2. Outdoor Thermal Comfort and Green Façades

4.2.1. Performance Determinants

Green façades impact OTC levels in outdoor environments. Green façade system properties are the primary factors exhibiting a direct relationship between the green façade system and OTC, while urban design parameters and climatic context are the secondary factors, with an indirect relationship. Green façade system properties comprise plant properties, type of plant, leaf area index, leaf angle, plant foliage thickness, percentage of wall coverage, emissivity of substrate, water coefficient of plant substrate, etc., which impact the thermal performance of the green wall [10,14,16,50].

The review highlights urban design parameters as one of two indirect factors impacting the thermal performance of green façades. These include façade orientation [14]; distance from the wall [13,14,49]. The other factor in these studies is climatic context, such as the tropical rainforest (Af) context [50]; the hot semi-arid (BSh) context of Shiraz, Iran [10]; the humid subtropical (Cfa) context of Guangzhou, China [16,50]; the temperate oceanic (Cfb) context [50]; the hot summer Mediterranean continental (Csa) context of Madrid, Spain [13,49,50]; and the hot summer humid continental (Dfa) context of Chicago, USA [14,50].

The findings of this review indicate that green façade systems help reduce PET levels. A comparative study of twenty façade materials highlighted that living walls showed the lowest PET levels and surface temperature (T_s) during the day [10]. The study also highlights that living walls, although expensive, can help mitigate glare problems due to high solar reflectance. Another study on green façades in transitional/shaded areas in the humid subtropical climate of Guangzhou, China, showed that the average PET was reduced by 2.54 °C and 1.43 °C compared to the unshaded area [16].

Moreover, similar results have been observed for microclimatic meteorological variables. For instance, a review study of vertical greening in historic urban context highlights its advantages thanks to its characteristics of reducing fluctuation in microclimate variables, namely surface temperature (T_s), air temperature (T_a), relative humidity (RH), solar irradiation (SR), and particulate matter (PM) [50]. Another study with similar variables in an urban context of a Mediterranean climate showed reduced fluctuations in air temperature (T_a), relative humidity (RH), and solar irradiance (SR) measured at four distances from the green wall [49]. A different climatic context, namely, the humid continental climate of Chicago, showed similar results. The research exploring the effect of a ~20 cm thick layer of Boston ivy (Parthenocissus tricuspidata) on environmental conditions demonstrated that the ivy reduced the surface temperature (T_s) by an average of 0.7 °C and the outdoor air temperature (T_a) by an average of 0.8–2.1 °C depending on the façade orientation [14]. A similar field measurement study of green vs bare walls in the cold semi-arid climate of Madrid, Spain, showed that green walls help with reducing temperatures in outdoor microclimates [13]. The study also demonstrated that reduced air temperature (T_a) in summer was about 2.5–2.9 °C, while in autumn the record was 1.5 °C.

4.2.2. Models/Metrics

The performance indicators used to understand the impact of green façades on the OTC include the OTC metric, namely, PET [10,48]; and meteorological variables such as surface temperature (surface T) [10,14,50], globe temperature (globe T) [48]; solar reflectance (R_s) [10]; air temperature (air T) [13,49,50]; relative humidity (RH) [48,49,50]; solar irradiation (SR), [50]; wind velocity (W) [48]; and mean radiant temperature (MRT) [48].

4.2.3. Green Façade Types

Green façades are typically categorized into two types based on their design and percentage of coverage—(a) living wall systems with modules, substrate for plants, and a water system (irrigation/drainage) [10,13,49], and (b) green walls with climbing plants on façades [14,16]. These are referred to as ‘continuous living walls’, ‘modular living walls’, ‘green walls’, ‘vertical green system, ‘ivy façade’, etc. [16,18] in the literature. Besides the system design, plant types also play a significant role in their performance with respect to OTC. The list of plant types studied here comprises: Heuchera americana “dale’s strain”; Sedum acre “golden carpet; Sedum album “coral carpet”; Thymus communis; Lonicera nitida “maigrun”; Heuchera americana “palace purple”; Carex oshimensis; Delosperma cooperi; Gazania rigens; Thymus vulgaris; and ivy façade with a 20 cm thick layer of Boston ivy (Parthenocissus tricuspidata) [14,49].

4.3. Outdoor Acoustic Comfort and Façade Materials

4.3.1. Performance Determinants

The acoustic performance of building façades plays a crucial role in shaping urban soundscapes by influencing noise levels and enhancing outdoor acoustic comfort. Various studies have examined the effects of different façade materials, geometries, and design strategies on sound absorption, reflection, and diffusion in urban environments. The primary and secondary factors exhibiting the relationship between materials and OAC are material properties and façade design parameters. The material properties in this research are direct factors that impact the acoustic comfort and include sound absorbance (sound reflection) [51,54] and reflectivity [19]. The façade design parameters are the indirect factors impacting the acoustic performance of an envelope system and include façade geometry details [51,56], corner cube retroreflective facades [55], louvres [8], exposed vs shielded facades [8], façade height [53], perceiver location [19], and balcony design [8,19,51,54].

Façade design parameters include geometric configurations that significantly impact noise attenuation, with research indicating that incorporating sound-absorbing cladding can reduce the SPL by up to 10 dB on the treated façade and 3 dB on the opposite façade. This highlights the importance of early integration of shape and material optimization in achieving acoustic comfort [51]. Additionally, different materials exhibit varying reflective properties, with clinker brick and mineral plaster producing higher sound reflections compared to HPL hardboard, which has lower reflection values by up to 0.8 dB at a height of 2 m [52].

The role of façade geometry is further emphasized in studies analyzing retroreflection effects, where specific geometries, such as corner cube arrays, resulted in increased retroreflection levels by 5–20 dB compared to flat surfaces, particularly in the 2000–4000 Hz frequency range [55]. These findings underscore the significance of façade geometry in redirecting sound energy and enhancing acoustic comfort in urban environments. Furthermore, a systematic review of building envelope strategies identified that textured façades and sound-absorbing materials could achieve noise-reduction levels ranging from 5.8 dBA in exposed street canyons to 7.3 dBA in shielded areas, with elements such as balconies and louvres contributing to improved sound diffusion and absorption [8]

Experimental research in university courtyards has demonstrated that taller façades with reflective materials, such as concrete and large glass surfaces, resulted in prolonged reverberation times, reducing speech clarity and intelligibility, whereas shorter façades with diffusive materials showed improved acoustic conditions [53]. Similarly, noise mitigation studies in urban street canyons found that materials such as pumice stone and refractory brick provided up to 5.1 dB insertion loss at 5 kHz, while cork wood and metal wool offered lower noise-reduction capabilities, emphasizing the importance of selecting materials based on their frequency-dependent performance [54].

Balcony design is also critical in OAC. For instance, a study shows that balconies behave acoustically like coupled cavity resonators that trap and dissipate part of the incident sound energy [65]. A psychoacoustic study further emphasized that moderate façade absorption (e.g., αw ≈ 0.55) resulted in optimal outdoor acoustic comfort, while overly reflective or highly absorptive materials caused discomfort. The study highlighted the importance of façade absorption properties in combination with balcony design and sound sources in shaping perceived comfort levels [19]. Recent advancements in computational methods, such as rule-based frameworks and Boundary Element Method (BEM) simulations, have enabled dynamic adaptation of façade geometries to acoustic constraints, revealing that curvilinear surfaces produce better sound scattering compared to planar designs. This approach allows for scalable and efficient acoustic evaluations in urban planning [56].

4.3.2. Models/Metrics

The performance indicators used to understand the impact of façade design on the OAC include sound-pressure level (SPL) [51,53], A-weighted equivalent sound level (LAeq) [52,54], reverberation time (RT) [53], early decay time (EDT) [19,53], speech definition index (D50) [19,53], Rapid Speech Transmission Index (RaSTI) [53], sound-absorption coefficient [54], early reflection SPL [55], noise abatement measured through insertion loss [8], speech transmission index (STI) [19], speech clarity index (C50) [19], music clarity index (C80) [19], lateral energy fraction (LF80) [19], perceived comfort [19], reflected sound pressure [56], and noise reduction [8].

4.3.3. Material Types

Types of materials explored in OAC include glass [51,53], brick [52,54], plaster [51,52], high-pressure laminate (HPL) [52], concrete [53], pumice stone [54], cork [54], metal wool [54], foam [54], nylon and polyester compound [54], and stainless steel sponge [54]. A comparative analysis of multiple materials conducted in narrow vertical strips (comprising only 8–17% of the wall area) made of porous pumice or refractory brick achieved a 3–5 dB insertion loss across the 1.25–5 kHz band. In contrast, the untreated hard wall provided virtually no mitigation [54]. Another study showed that a rigid laminate (HPL) panel added only 0.8 dB of reflection at 2 m from the wall. In contrast, mineral plaster and clinker brick returned 2.5 dB and 2.8 dB, respectively, confirming that HPL behaves as the most acoustically “soft” of the three claddings [52].

The impact of façade materials on urban acoustics is multifaceted, involving a combination of material properties, geometric configurations, and strategic design interventions. Studies have shown that sound-absorbing materials, geometric adaptations, and integrated design approaches can significantly improve acoustic comfort in urban environments. Future research should continue exploring innovative materials and computational techniques to enhance the acoustic performance of urban façades.

4.4. Outdoor Acoustic Comfort and Green Façades

4.4.1. Performance Determinants

Similar to OTC, green façade system properties serve as the primary factor for OAC performance and comprise plant type, leaf area index, leaf area density, angle of leaf orientation, growing medium, coverage, substrate design (type, thickness, and moisture content), structural design, construction materials, and air layer [15,57,58,60]. For instance, a recent review study has demonstrated the potential to reduce noise levels by up to 40 dB, depending on system design, plant type, and substrate composition [57]. Other studies have highlighted the effectiveness of green façades in reducing noise through variations in vegetation density, substrate properties, and structural design. Green walls with moderate greenery coverage (65–80%) and optimized structural support systems, such as recycled wood-plastic composites, provide significant noise-reduction benefits, particularly in the low-to-mid-frequency range (~1000 Hz) [15] Substrate thickness plays a crucial role in absorption, with thicker substrates achieving noise reductions of 5–10 dB, whereas thinner substrates result in lower reductions (2–3.9 dB) [58]. The presence of specific plant species, such as Bergenia cassifolia and Jacobaea maritima, enhances absorption, with larger leaves contributing more effectively to mid-high frequencies and smaller leaves performing better at high frequencies [59].

Research also indicates that indirect green façades, installed with an air gap between the wall and vegetation, perform better than direct installations, achieving up to 2 dB greater noise reduction [57]. Additionally, studies have highlighted that the moisture content in the substrate can negatively impact performance by filling air pockets and reducing overall absorption efficiency [15]. Field studies have confirmed that modular green wall systems can improve sound absorption across all octave bands, with the best performance observed at mid to high frequencies [60]. Moreover, studies have shown that adding water to the growing medium can negatively impact noise absorption, as it fills the air pockets that contribute to acoustic attenuation [58]. Green walls have demonstrated an absorption coefficient of up to 0.40 in laboratory tests, with the best results seen in the mid-to-high frequency range [59]. In addition, studies conducted in street canyons suggest that full façade coverage is not necessary to achieve effective noise reduction; partial coverage can provide similar results with lower installation costs [57].

On the other hand, the secondary factor design parameters contributing to noise reduction include canyon width [57,60], distance from the source [58], and façade width, building heights, water needs, wall types [60]. For instance, studies have shown that green façades and living wall systems (LWSs) have emerged as practical solutions to mitigate urban noise pollution, a significant challenge that impacts human health and well-being. While green walls offer notable acoustic benefits, their effectiveness varies depending on environmental conditions and system configurations. The cost-effectiveness of these systems for noise mitigation alone has been questioned, emphasizing the need for optimized material selection and design improvements to enhance performance. Furthermore, green façades are most effective in narrow street canyons where they can reduce noise reflections and contribute to overall urban comfort [60].

4.4.2. Models/Metrics

The most common performance metric for acoustic was found to be noise reduction (NR) [57]. This was followed by sound-reflection levels (SRL) [57], sound-absorption coefficient (SAC) [15,57,58,59], noise absorption at different frequencies [58,59], sound attenuation (SA) [59], and sound reduction index (SRI) [59].

4.4.3. Green Façade Types

Green façades in this section have been categorized into (a) living wall systems and (b) green walls [57]. While the substrate design and its construction have been identified as contributing factors, the plant type also plays an essential role in its performance. The list of plant types studied in this scope includes Jacobaea maritima [15,59], Hedera helix [58], and Bergenia cassifolia [57,58], and Helichrysum thianschanicum [59]. Overall, green façades are a promising solution for mitigating urban noise, providing measurable reductions across various frequency ranges. To maximize their potential, future research should focus on optimizing substrate properties, vegetation types, and structural configurations to achieve improved acoustic performance and cost efficiency.

4.5. Outdoor Visual Comfort and Façade Materials

4.5.1. Performance Determinants

Envelope materials play a significant role in OVC, and material properties are a direct factor that interacts with the occupants and impacts their OVC. These include light reflectance [61,62], material colors [63,64,66], and daylight absorption [9]. Other design parameters include age and gender [61], sky conditions (rainy and clear skies) [63], eye movement [64], and sky view factor [9].

The study of these factors centers on key issues such as glare, visual fatigue, and reduced daylight availability. For instance, studies show that glare often hinders clear vision, causing irritation [9]. Building façades significantly influence pedestrian visual comfort by affecting material reflectance and daylight illuminance [62]. The authors of this survey study explored the reflectance of clear versus dark façades in Fribourg, Switzerland. They found that high-reflectance materials contributed to extreme glare in bright outdoor conditions [62], highly reflective materials on glazed envelope [61], and surfaces with different color attributes [64]. Another survey on highly reflective materials on glazed envelopes showed that reflective façades significantly impact visual discomfort by increasing glare duration and prompting avoidance behavior among pedestrians. The study also showed that males exhibit higher avoidance responses, while females are more sensitive to glare [61]. Thus, field measurements and user surveys, as research methods, have contributed to understanding the role of façade material properties in glare perception and brightness levels. To enhance outdoor visual comfort and reduce glare-related discomfort, incorporating effective shading strategies is recommended.

In addition to reflectivity, façade color significantly influences outdoor visual comfort, especially under varying weather conditions. Clear weather conditions resulted in higher comfort ratings compared to overcast and rainy conditions, highlighting the importance of color selection in optimizing visual comfort [63]. Eye-tracking studies have shown that highly saturated or extremely bright colors increase cognitive load and visual fatigue [64]. Excessively bright or saturated façades can contribute to visual discomfort and increased pupil dilation, underscoring the importance of balanced color application in façade design.

Daylight availability and distribution are also crucial for visual comfort in urban outdoor spaces. Studies have shown that façade materials and urban geometry have a significant influence on daylight distribution. In urban areas, the sky view factor (SVF) also plays a critical role [9]. Higher SVF values enhance daylight penetration, improving visual comfort in less dense areas, while compact layouts with low SVF restrict daylight access, leading to visual discomfort [9]. However, research on the specific impact of building façades on outdoor visual comfort remains limited.

4.5.2. Models/Metrics

Various performance indicators have been identified in the OVC literature. These include cylindrical illuminance [62], specular façade reflectivity (SFR) [61], perceived glare [61,62], luminous sensation mean vote [62], discomfort glare (DG) [62], occupant behaviors (wearing sunglasses, moving hands above eyes, rotating or bending head, and blinking [62], gaze, and swipe [64]), perceived visual comfort (PVC) [63], luminance contrast (VC) [63], chromaticity contrast (CC) [63], color attributes (hue, saturation, brightness) [64], daylight glare index [9], horizontal illuminance, IES luminance ratios, daylight glare probability (DGP) [66], and daylight glare index (DGI) [9,66]. While the daylight factor is one of the common indicators of visual comfort, limitations persist in its accuracy, as it does not consider the dynamic outdoor variations of daylight environments [63]. While some studies have focused on one metric, others have investigated multiple metrics to assess visual comfort. For instance, the study in Idaho, USA, explored multiple metrics, namely horizontal illuminance, vertical illuminance, IES luminance ratios, DGP, and DGI, and found vertical illuminance to be the best metric [66]. Another study using cylindrical illuminance sensors identified highly reflective materials as major contributors to excessive glare, especially in high-illuminance areas [62]. One of the key outcomes of this study underscores cylindrical illuminance as a tool to analyze the geometry of the “open space”-(the space offering the largest free sky view) and is calculated as

E_cyl = π2 × (f_sky × L_sky + f_buildings × L_buildings + (1 − f_sky − f_buildings) × L_ground)

where: L_sky, L_buildings and L_ground are the luminance of the sky, the buildings, and the ground; f_sky and f_buildings are “form factors” giving the proportion of the sky and the buildings as “seen” from the specific location. Both values fall within the range of 0 to 1.

Overall, the role of building façades in shaping visual comfort in outdoor spaces is multifaceted. Key considerations include reflectivity, color attributes, material properties, and shading strategies. Compared to thermal and acoustic comfort, visual comfort remains underexplored in façade research. More importantly, in this pool of studies, no research was found on green façades, making it a key research gap in this topic. Thus, compared to thermal and acoustic comfort, visual comfort remains underexplored in façade research.

Most studies rely on small-scale experiments or user surveys, with a limited application of physiological tracking tools. Future research should incorporate eye-tracking, EEG-based discomfort monitoring, or VR-based visual simulation to understand how façade color, reflectivity, and geometry influence outdoor visual experience, especially under varying climate and lighting conditions.

Summary of results:

This research offers an in-depth understanding of the latest studies on materials and green wall treatments for façade design, focusing on OTC, OAC, and OVC. Figure 2 is a visual representation of these findings, structured around the three levels of analysis: Performance Determinants, Models/Metrics, and Types. The study’s focus is limited to façade treatment at the building level, which impacts the overall outdoor environment at the urban level.

One of the key findings of this research is the inventory of various factors under each section of the three analytical levels considered for the analysis. This inventory can be used to either develop new materials or assess existing materials in relation to all subsequent categories, rather than just one category. For instance, the performance of brick cannot be evaluated solely by its material properties. It is advisable that the other two categories, namely design parameters and the climatic context, also be considered. Thus, there is an underlying interrelationship between these features of the Performance Determinants. Similarly, extending this further, one must also look into the models/metrics associated with a particular material type.

5. Discussion

This review underscores the critical, yet often underappreciated, role of building façades in shaping OEC. While traditional façade research has primarily focused on indoor environmental quality and energy efficiency, this study shows that façade materials and vegetative treatments greatly impact the thermal, acoustic, and visual quality of nearby outdoor spaces [5]. Through the analysis of 41 peer-reviewed studies, we offer a comprehensive synthesis of the measurable and multifaceted contributions of façades to urban microclimates.

Among the reviewed studies, 24 examined the impact of façade materials or green façades at the urban scale, focusing on urban blocks or neighborhoods. For example, Tabatabaei and Fayaz investigated the effects of 20 façade treatments in a neighborhood in Shiraz [10]. At the building scale, 17 studies focused on immediate surrounding spaces and explored how façade treatments influence OEC. For instance, Wang et al. studied the impact of façade color materials on visual comfort in Shanghai [64]. While building-scale studies have shown façade effects on the immediate microclimate, Prieto and Pastén’s “urban acupuncture” approach illustrates that these localized benefits can build up over blocks and neighborhoods. Therefore, we can infer that the interactions between buildings and urban outdoor environmental quality (OEQ) are likely influential in shaping the broader urban–OEQ relationship [67].

To consolidate the review findings and highlight the interaction between façade strategies and outdoor comfort outcomes, Figure 3 presents a conceptual framework. It maps the relationship between two main façade types—material-based and green systems—and the mechanisms through which they influence OTC, OAC, and OVC. These mechanisms include high reflectivity, evapotranspiration, sound absorption, shading, and energy dissipation. By visualizing this interaction, the diagram emphasizes the need for integrative façade design strategies that consider cross-domain impacts.

In the domain of OTC, high-reflectivity materials, light colors, and low thermal mass have demonstrated the capacity to reduce surface and air temperatures, mitigating the intensity of the UHI effect [6,11]. For example, materials such as limestone and retroreflective coatings reduced thermal load more effectively than conventional materials like brick or concrete [10]. Green façades, especially those with continuous vegetative cover, lowered surface temperatures by 2–3 °C [10,14] and contributed to average PET reductions of up to 2.5 °C [16]. However, performance varied across climates and façade orientations, emphasizing the context-dependent nature of these strategies [45,50].

Regarding OAC, studies found that façade geometry and the material’s sound-absorption coefficients significantly shape urban noise propagation [52,54]. Retroreflective designs and textured surfaces, such as corner cube arrays or clinker brick, redirected or dispersed sound energy more effectively than flat, reflective façades [52,55]. Green wall systems—especially indirect installations with air gaps—achieved noise reductions of up to 40 dB across specific frequency ranges, making them a powerful dual-function intervention for both aesthetics and comfort [57]. Nonetheless, performance was influenced by factors such as substrate thickness, plant type, moisture content, and installation method [15,59].

In terms of OVC, the literature remains relatively sparse but offers important insights. Materials with high specular reflectivity contributed to glare and behavioral avoidance among pedestrians, particularly under clear sky conditions [61,62]. Studies noted that façades with highly saturated or overly bright finishes increased visual fatigue and pupil dilation [64]. Conversely, strategic shading devices, moderate color palettes, and attention to the sky view factor (SVF) were associated with improved luminance balance and reduced discomfort [9]. While less researched than OTC or OAC, OVC requires increased attention given its implications for user experience and public space quality.

Importantly, our review also identifies a need for cross-domain evaluation, as some façade strategies benefit one domain while potentially compromising another. For instance, while high-albedo surfaces improve OTC by reducing heat gain, they may worsen OVC by increasing glare [68] Similarly, green façades improve both acoustic and thermal comfort but require maintenance and are highly climate-sensitive [55]. These findings necessitate integrative assessments of façade performance, which account for the synergies and trade-offs among thermal, acoustic, and visual metrics.

Quantitative comparisons across studies further illustrate the value of multi-functional façade interventions. Green walls demonstrated temperature reductions of 2–3 °C, noise attenuation of up to 40 dB, and absorption coefficients as high as 0.40 in laboratory settings [59]. Retroreflective façades improved thermal indices like WBGT and UTCI more effectively than diffuse reflective materials [11]. While exact values varied due to geographic, geometric, and climatic differences, these ranges highlight the measurable potential of façade treatments for enhancing OEC [38].

Although these promising findings are encouraging, several limitations are apparent. Our study shows that the geographic distribution of research is heavily concentrated in temperate and arid climates, with fewer assessments in tropical, high-humidity, or high-latitude areas. [34,50]. This limits the generalizability of design recommendations. Moreover, the studies rely on varied comfort indices and modeling tools, making cross-study comparisons difficult [21]. Furthermore, since green façades have shorter lifespans compared to hard materials, we recommend investigating the sensitivity analysis for the durability of various systems [57,59]. However, since the performance and the environmental impact of the same remain higher, it still serves as a sustainable solution for green façades.

To address these gaps and support more robust design practices, we recommend a multi-pronged research agenda. Future work should pursue interdisciplinary methods that simultaneously model thermal, acoustic, and visual comfort outcomes. Long-term field studies are needed to validate simulation results and capture seasonal or maintenance-related effects, particularly for vegetative systems. Additionally, certain studies in the OTC domain have explored advanced computational techniques, such as machine learning and AI-driven generative design for predicting statistical indices [69]. Such approaches can be extended to predict OEC outcomes based on façade properties, providing designers with faster and more holistic performance feedback during early planning phases.

Beyond academic implications, integrating OEC into façade design supports broader sustainability goals. By aligning with policy frameworks such as the United Nations Sustainable Development Goal 11 (Sustainable Cities and Communities) and green building certifications (e.g., LEED, WELL), these strategies can help cities create healthier, more inclusive environments [70,71]. Simulation tools and decision-support systems should advance to incorporate outdoor comfort metrics, in addition to traditional energy and cost factors, to facilitate evidence-based design and ensure regulatory compliance [30,69].

While individual studies often focus on one domain of comfort, façade strategies usually affect multiple parameters simultaneously. Integrated design approaches must weigh the trade-offs to deliver holistic outdoor comfort. The conceptual model presented (Figure 3) supports such assessments. This review advocates for a paradigm shift: from façades as static barriers to façades as dynamic, multi-functional elements that actively shape microclimate, comfort, and public well-being. Through thoughtful material selection, geometric consideration, and vegetative integration, façade design can serve as a powerful lever in building not only energy-efficient but also climatically responsive and socially inclusive urban environments.

6. Conclusions

This review examined 41 peer-reviewed studies published from 2010 to 2024 to assess how façade materials and green systems influence OEC. The study confirms that building façades are not just architectural elements for enclosure or aesthetics but active components that affect outdoor environment quality in thermal, acoustic, and visual aspects.

Out of all the studies, 24 focused on urban-level analysis, while 17 examined individual buildings. This trend highlights a growing focus on the interaction between building façades and OEC. Moreover, the OTC domain was found to be explored more than the OAC and OVC, highlighting further research potential in the latter two domains, particularly OVC. A major finding of this research is the necessity for a broad cross-domain study encompassing OTC, OAC, and OVC, as most existing studies tend to focus on just one domain. Therefore, we developed a conceptual framework to incorporate façade materials into the OEC domains.

Another important finding of this study highlights how various features of the analytical structure (performance determinants, metrics/models, and material/green façade types) are interrelated and could significantly impact the OEC. Specific findings in this area are briefly presented below:

• OTC is impacted by material properties distinctly with close connections to climates and design parameters. High-reflectivity materials, light colors, and low thermal mass could reduce surface and air temperatures, thus mitigating UHI intensity. Similarly, green façades could lower surface temperatures and contribute to average PET reductions around 2.5 °C.
• OAC is impacted by design parameters besides material properties. Façade geometry and the material’s sound-absorption coefficients significantly shape urban noise propagation. Green wall systems, while reducing noise up to 40 dB, also serve as powerful, dual-function interventions that enhance both aesthetics and comfort.
• OVC, an under-researched area, shows a close association with material properties and design parameters. Certain materials with high specular reflectivity and saturated finishes can contribute to glare, visual fatigue, and pupil dilation, which can be addressed through proper design.
• One façade treatment (façade material or green façade) could impact all three comfort factors related to OEC, but trade-offs must be evaluated. For instance, high-albedo surfaces could improve OTC by reducing heat gain but worsen OVC by increasing glare.
• Limited work on OEC and façades is available in tropical, high-humidity, or high-latitude regions. Since OEC is climate- and context-specific, additional research is required to cover variations in both design conditions.

The analytical structure of this review paves new research pathways exploring the quantitative interrelationships between the three elements (performance determinants, metrics/models, and material/green façade types). All in all, this review contributes to the existing knowledge on building façade design and its impact on the urban fabric. As metropolitan areas face increasing challenges from climate change, optimizing the performance of building façades is both a design imperative and a policy opportunity. Aligning façade strategies with global frameworks such as the UN SDGs can enhance the livability and resilience of cities. Designers and planners are thus encouraged to consider building envelopes as part of the larger urban ecosystem.