A critical review for formulation and conceptualization of an ideal building envelope and novel sustainability framework for building applications

1. Introduction

Energy consumption is increasing globally due to population growth and improvement in living styles due to economic development. This is also resulting an increase in greenhouse gas (GHG) emissions (Kumar et al., 2018a). Therefore, energy savings and environmental protection are essential for achieving sustainable development goals. According to U.S. Energy Information Administration (EIA) report (2021), the building sector consumes 39% of total energy, followed by 33% of the industrial sector and 28% of transportation. Approximately 54% of total building energy consumption is in residential buildings, and the rest is consumed by commercial buildings (Dunn, 2022). The building sector is responsible for one-third of GHG emissions in the world (Kumar et al., 2020a).

In building, heating, ventilation, and air condition (HVAC) system accounts for 42–68% of total building energy consumption, followed by 14–26% for the domestic hot water system, and 16–32% for lighting and appliances (Pérez-Lombard et al., 2008). The energy consumption of HVAC system and lighting depends on building envelope design (Gaetani, 2019), whilst energy consumption of other components relies on their inherent design characteristics and occupant behavior (Borg and Kelly, 2011). The use of energy efficient appliances reduced residential building energy consumption without changing peak energy consumption (Borg and Kelly, 2011). HVAC system used energy to maintain acceptable thermal comfort depending on heat and mass transfer through building envelope (Kumar et al., 2022a). Heat transfer accounts for 50–60% of the total energy used in HVAC system (Kumar et al., 2018b). Lightings maintain visual comfort in a building which depends on window-to-wall ratio, and window properties (Alghoul et al., 2017).

Many studies were conducted to reduce energy consumption of HVAC system and lighting by designing an energy efficient building envelope considering several parameters. For instance, increasing building envelope thermal resistance, thermal storage and solar absorptivity reduced HVAC system energy consumption by 20–80% (Kumar et al., 2020a), 35–56% (Hamidi et al., 2021) and 22–46% (Peoples et al., 2022), respectively, depending on climatic conditions. Similarly, upgrading window type and its glazing reduced lighting load and HVAC energy consumption by 39–56% (Charles et al., 2019) and 17–47% (Wang and Greenberg, 2015), respectively. The insulation materials, phase change materials (PCMs), and coating materials are the key component of an opaque envelope. They ensure energy conservation and thermal comfort in buildings, whilst visual comfort depends on the glazing properties of the transparent envelope (G et al., 2018). Buildings need to conserve energy, ensure an acceptable level of thermal comfort, acoustic comfort, visual comfort, and improve indoor air quality. Building energy conservation depends on several parameters, including design, indoor and outdoor conditions, and selection criteria, as seen in Fig. 1.

Previous review studies focused on thermophysical properties of building materials (structural (Latha et al., 2015), insulation (Kumar et al., 2020a), phase change materials (Ikutegbe and Farid, 2020), coating (Santamouris and Yun, 2020), and glazing (Wong, 2017)) properties. Measure their performance in terms of energy, economy, environment, and thermal comfort by classifying as conventional, state-of-the-art and sustainable material (Ikutegbe and Farid, 2020) and its performance was evaluated considering the climatic condition. Building envelope design depends on numerous design parameters and selection criterion to design energy efficient building in accordance with the need and will of stakeholders. For instance, public stakeholders were more concern about energy-efficient and eco-friendly buildings than occupant satisfaction and cost. However, private stakeholders' compromised on energy efficiency and emissions by improving occupants' comforts with economic incentive (Ascione et al., 2019c). Heritage buildings' stakeholder preserved the artwork and cultural values of building instead of energy efficiency, eco-efficiency, and visitors' satisfaction (Yılmaz and Yılmaz, 2020a). The occupant satisfaction also affected building envelope design. For instance, residents and visitors seek thermal comfort in residential buildings and hotels (Yılmaz and Yılmaz, 2020b). The productivity of office occupants and industry workers depend on visibility in their working space (Leung et al., 2020). Travelers seek acoustic comfort in bus stations, railway stations, and airports (Yang and Moon, 2019). Moreover, patients need better indoor air quality in hospitals along with Medicare (Khalid et al., 2019). The occupant satisfaction is the measure of productivity of offices occupants, which is increased by 5–15% by improving lighting conditions (Amasyali and El-Gohary, 2016) and reducing lighting power by 25% (Akram et al., 2021) by optimizing window-to-wall ratio. Occupants were comfortable at higher set-point temperature in Green Energy Office (23.75 oC) than Low Energy Office (22 oC). Occupants were unsatisfied in a passive office in a Tropical climate in the absence of mechanical cooling (Qahtan et al., 2010). There is a need to understand the influence of design parameters, stakeholder's involvement and occupant's satisfaction on building envelop design.

This review study aimed to explain the influence of different parameters, conditions, and selection criteria on building envelope design and propose a conceptual framework to design a sustainable building envelope. The specific objectives are to.

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Review and classify building envelope design considerations
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Understand the impact of design considerations on building envelope performance
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Formulation and conceptualization of an ideal building envelope and sustainability framework

This paper is organized by classifying design considerations and their parametric analysis before discussing the formation and conceptualization of an ideal building envelope and sustainability criteria. Section 2 classifies building envelope design considerations as design parameters, indoor and outdoor conditions, and evaluation indicators. Section 3 describes the influence of design parameters (building types, building geometry, building envelope type, and building materials) on building performance (energy, environment, economy, and occupant satisfaction). It further classifies building materials according to structure, insulation, phase change materials, and coatings. It also discusses indoor environment requirements and weathering parameters' impacts on building performance. Then, it discusses the proposed conceptual framework for selecting a sustainable building envelope in section 4. Section 5 presents potential future research directions. Finally, the present study outcomes are concluded in section 6.

2. Classification of design considerations

An energy-efficient envelope design aims to reduce building energy consumption and provide thermal comfort to occupants considering several design variables such as geometric, envelope material properties and dimensions, air conditioning system, and internal load. However, these variables are inextricably interrelated from the building envelope thermal optimization point of view. Optimization of opaque envelope requires studying construction materials’ properties wherein the solar absorptivity, moisture permeability, and type of insulation are more influencing thermal variables (Yang and Santamouris, 2018). Fig. 2 classifies different influencing factors which have a significant impact on building envelope design and their corresponding description is presented in Table 1.

Table 1. Description of parameters considered in thermal optimization of building envelope.

Parameters	Description	References
Geometry
Building type	According to their use, buildings are classified as residential, commercial, and industrial.	Feng et al. (2019)
Shape Factor	It measures the external surface area to the volume of occupied space.	Fantucci et al. (2019)
Orientation	Building's position relative to seasonal variation in sun's path and wind patterns.	Ramin et al. (2016)
Solar shading	It controls solar heat gain and light admittance in the buildings.	Wati et al. (2015)
Structure type
Opaque envelope	Building envelope without windows	Ozel (2011a)
Glazed envelope	Building envelope with windows	Huang et al. (2019)
Material type
Construction	Naturally occurring substances such as sand, clay, rock, and wood are used to construct buildings.	Kumar et al. (2020b)
Insulation	Materials resist heat transfer through the building envelope.	Kumar et al. (2020a)
PCM	It stores thermal energy by changing its phase.	Kumar et al. (2022c)
Coating	A layer used to avoid heat	Triano-Juárez et al. (2020)
Indoor Parameters
Occupancy	Many people live or use building space.	Lu et al. (2020)
Set-point temperature	Keep indoor air temperature at a user-defined value to maintain thermal comfort.	Ozel (2016)
Relative humidity	It is a percentage of water vapor in indoor air at a given temperature to its saturation condition. Its acceptable thermal comfort range is 40–60%.	Yamankaradeniz (2015)
Outdoor Parameters
Climate	It measures the atmospheric conditions of a location over a long period.	Kameni Nematchoua et al. (2017)
Temperature	It measures the degree of hotness and coldness of ambient air.	Pásztory et al. (2018)
Relative humidity	It is a percentage of water vapor in ambient air at a given temperature to its saturation condition.	Pásztory et al. (2018)
Solar absorptivity	It measures the ability of the exterior envelope material to absorb incident solar radiation.	Kontoleon and Bikas (2007)
Wind speed	It is a measure of velocity with which wind blows	Yildiz and Ersöz (2016)
Evaluation Parameters
Energy	Lifecycle energy analysis includes embodied energy and operational energy.	Jie et al. (2018)
Economy	Lifecycle cost analysis is the sum of additional investment and the time value of operational energy.	Jie et al. (2018)
Environment	Lifecycle environmental analysis considers the equivalent embodied carbon and operational GHG emission from a building.	Jie et al. (2018)
Thermal Comfort	It measures an acceptable level of indoor operative temperature and relative humidity.	Yılmaz and Yılmaz (2020c)
Visual Comfort	It is a subjective reaction to the quantity and quality of light within any given space at a given time.	Yılmaz and Yılmaz (2020c)
Other	Technical, social, cultural, and aesthetic	Xin et al. (2014)

3. Parametric analysis

3.1. Design parameters

Building design team designs building for service requirements in accordance with ambient conditions. Design parameters should comply with legislation and energy targets. The building service engineers relate building operating conditions with design parameters with reference to regulations, codes, and standards. The building energy efficiency significantly depends of design parameters such as building type, building geometry, and building envelope (Preet et al., 2022). The energy consumption and savings potential of building design parameters are discussed below:

3.1.1. Building type

Fig. 3 shows the envelope thermal resistance and energy use in baseline and retrofitted building types. A building with low thermal resistance consumes more energy than a high resistive material envelope. Moreover, reference buildings with little difference in thermal resistance, such as the ancient villa (Ascione et al., 2019b), and industrial buildings (Ascione et al., 2020c) are the most energy-intensive, consuming over 200 kWh/m2-year followed by the school with below 150 kWh/m2-year (Moazzen et al., 2020a) and residential historic (Ciulla et al., 2016). Single-family houses consume 100 kWh/m2 less energy annually than offices at given thermophysical properties and climatic conditions (D'Agostino et al., 2019), highlighting the building usage. The duplex house consumed 50% more energy than a single story house and single apartment under climatic conditions of Tripoli, Libya (Dekam et al., 2020). However, industrial buildings, schools, and ancient villas have similar energy consumption levels due to heavy equipment, high occupancy, and extended external surface area. Conclusively, consideration of building type is essential for preliminary design and retrofit since residents are more concerned about thermal comfort, whilst workers need both thermal and visual comfort and energy supply as per the type of building.

3.1.2. Building geometry

Building geometric parameters are the measure of length, width, and height in accordance with building shape, orientation, and shading level (Liang et al., 2021).

3.1.2.1. Shape factor

Heat absorption is a function of building volume, whereas heat transfer depends on the surface area. Therefore, the ability of a building's indoor environment depends on the shape factor, which is the ratio of the external surface area to the volume of the building. An energy-efficient building should have the least shape factor (compactness) (Fantucci et al., 2019). The impact of the building shape factor on primary energy use and cost savings in reference and retrofitted buildings is illustrated in Fig. 4 (Ciulla et al., 2016). Depending on ambient conditions, the shape factor increases primary energy use and lowers the cost savings for different retrofit strategies (Khoukhi, 2018). For instance, low shape factor buildings consume less energy in cold climates than in mild climates (Depecker et al., 2001). The square shape building uses 8–27% less energy than an elongated shape one by orienting square shape house facing four cardinal and the elongated house to the south side only (Alanzi et al., 2009). L-shape unit shifts peak air conditioning load by 2 h on either side of noon, benefiting economic incentives (Hachem et al., 2011). Buildings have an optimum shape factor of 3.5 in Ankara, Turkey, with minimum heat transfer through the building envelope.

3.1.2.2. Orientation

The impact of building orientation on energy use in buildings under different climatic conditions is illustrated in Fig. 5a–b (Aksamija and Peters, 2017). Energy consumption does not change with building orientation in case of opaque envelope because energy consumption depends on heat transfer through building envelope. Heat transfer through building envelope does change due to uniform temperature difference in a climate and constant thermal resistance of the building envelope. Changing climatic condition changes energy consumption of both opaque and glazed wall buildings because of change in temperature difference. For instance, buildings in cold climates has higher indoor and outdoor temperature difference resulting in high energy consumption of 10–12 GJ and 14–15 GJ in opaque and glazed envelope buildings, respectively. The orientation has significant impact on building with glazed walls. The building energy consumption is independent on climatic conditions in case of the East and West orientations. Buildings installed with window in North and South orientation have consumed 50% less energy than same building equipped with windows in East and West directions. They installed windows in a north-oriented envelope to reduce energy use in a detached house at given thermal resistance and without plot constraints. In case of window installation in east and west direction due to plot constraints, then energy consumption could be reduced by increasing wall thermal resistance, as seen in Fig. 6 (Daouas, 2011). A similar impact of wall orientation on energy-savings is investigated in Turkish (Ozel, 2011a), Cameroon (Kameni Nematchoua et al., 2017), and Iranian (Ramin et al., 2016) buildings considering window installation in the East and West walls.

3.1.2.3. Shade level

Fig. 7 exhibits the impact of shading level on thermal resistance corresponding to cost savings for different orientations. Shading lowers thermal resistance and cost savings potential of insulation materials. The impact of wall orientation disappears at higher shading levels (Wati et al., 2015).

3.1.3. Building envelope type

The building envelope is opaque and glazed depending on building type and climate (Taghizade et al., 2019). The opaque envelope includes solid layers of masonry, insulation, plastering, metal framing, and sometimes window. Curtain walls are glazed envelopes primarily constructed using transparent and translucent materials and metal framing. Thermally, both envelopes have different characteristics due to the material characteristics and construction methods. An opaque envelope has excellent heat retention and thermal comfort, while the glazed envelope allows more daylight with improved occupant visual comfort (Emmerich et al., 2005). For instance, Ajla and Troy (Aksamija and Peters, 2017) investigated thermal transmission and energy lossthrough seven wall structures for all US climates using THERM 6.3 and EnergyPlus 8.3 simulation tools, respectively. They noted that the curtain wall had the highest heat transmission, followed by a brick cavity wall. Also, a curtain wall with a thermally broken frame (TBF) was better than a brick cavity wall. In the case of Rainscreen, terracotta cladding (TC) should be used instead of glass fiber reinforced concrete (GFRC). The highest heat transmittance was observed for Rainscreen having terracotta cladding and thermal spacer and isolators (0.33 W/m2K) as seen in Fig. 8.

3.1.3.1. Opaque envelope

The opaque envelope is configured as single (Feng et al., 2019) and multi-layer structure (Cuce et al., 2014). The thermal performance of a multi-layer wall is better than a single-layer and air-cavity envelope structured using concrete blocks or solid bricks and load-bearing connectors to entrain air (Zhang et al., 2016). However, the thermal performance of the air-cavity envelope depends on the number of gaps (Seth et al., 1981) and its thickness (Yuan et al., 2017a; Cuce et al., 2014). The concrete/air/insulation/air/concrete configuration has better thermal performance than that of insulation/air/concrete/insulation (Seth et al., 1981). The air gap thickness should be kept below 5 mm to avoid convection current depending on climatic conditions (Šadauskiene et al., 2009). The optimum air gap thickness for walls and roofs is 20 and 19 mm, respectively (Alnahhal et al., 2018). Air gap reduces the load-bearing envelope capacity of the building, reducing mechanical strength (Feng et al., 2016; Feng et al., 2017). Thermal instability occurs due to seasonal variation in air density with ambient temperature. Thermal instability increases as convection current increases due to the global Rayleigh number. Therefore, the asymptotes approach optimizes thermal resistance per unit volume air cavity considering heat transfer, strength, and volume of the envelope (Xie et al., 2014). A wide air gap enables airflow resulting in a ventilated envelope (Ciampi et al., 2003). Both convective and radiative heat transfer takes place in a ventilated envelope. A well–designed ventilated wall dwindles cooling energy consumption by 40% (Ciampi et al., 2003).

From an energy and economic perspective, a single-layer envelope requires thick insulation materials than a multi-layer envelope (air cavity structure (Cuce et al., 2014) and sandwich wall (Kumar et al., 2020b)) for maximum savings. The single layer envelope has smaller thermal resistance than multi-layer envelope because air has lowest thermal conductivity than construction and insulation materials. Therefore, the multi-layer envelope requires smaller optimum insulation thickness than single layer envelope to achieve same envelope thermal resistance at given energy savings. Massive structures without air gaps improve building energy efficiency and thermal comfort in a hot climate (Al-Sanea et al., 2012), the Mediterranean climate, and the equatorial climate (Stazi et al., 2015). In contrast, light-weight and cavity structures are suitable for temperate and cold climates (Gregory et al., 2008). Furthermore, the stone and cavity wall structures are recommended for cooling (Ogoli, 2003) and heating-dominated regions (Gregory et al., 2008), respectively, for better thermal comfort. Hence, the air gap is most effective in the heating-dominated region due to its high insulating properties. At the same time, it is less effective in cooling-dominated regions due to poor thermal mass, recommending air gap envelopes for heating-dominated regions (Alhefnawi and Abdu-Allah Al-Qahtany, 2017). Air gap envelope reduces life cycle cost by 28–54%, and 65–77% in the Maldives, Malaysia, and Morocco (Mahlia and Iqbal, 2010).

3.1.3.2. Glazed envelope

Solar energy is a renewable energy source that benefits visual comfort and provides physiological relaxation to building occupants at circadian rhythm on energy cost (G et al., 2018). It is essential to understand a glazed envelope's thermal and visual performance considering the glazing area and glazing type (Huang et al., 2019). The impact of Window-to-wall ratio (WWR) on cooling and heating energy consumption of an office building with different orientation of windows is illustrated in Fig. 9a–b (Alghoul et al., 2017). In north oriented wall, increasing WWR has slightly increased cooling demand but dramatically decrease in heating load due to smaller solar energy transmission through window. Moreover, cooling energy consumption is increasing with WWR in all other directions, while heating energy consumption remained same in south oriented wall. Hence, it has smaller solar transmission with minimum illuminance over 2000lx reducing productivity of worker and working time, as seen in Fig. 10. For examples, the total energy use in glazed envelope building is 155 kWh/m2-year at 80% WWR and 25 kWh/m2 at 20% WWR applying solar coating on window without changing daylight (80%) in Santiago (Pino et al., 2012). However, the optimum WWR resulted in thermal discomfort of 10–20% with a decrease in lighting load of just 1.5–9.5% in a typical tropical office building (Pathirana et al., 2019). The optimum window size of a typical residential building would reduce primary energy use, predicted percentage dissatisfaction, and lighting load by 11.42%, 4.52%, and 4.94%, respectively, in a temperate-humid climate (Yılmaz and Yılmaz, 2020b).

A model-based control system satisfies visual constraints for HVAC system operation, lighting, and shading. Additionally, external shading drops the solar heat gain of a building with little impact on visual satisfaction. Shading length depends on window orientation, time, and climatic conditions. For instance, the West/East wall must have the longest shading, while the south/north has the shortest shading (Wu et al., 2017). As seen in Fig. 11, the triple-glazed window is the most energy-efficient due to its transmission and solar heat gain coefficients (SHGC) of only 0.68W/m2 K and 0.41, respectively. However, a triple-glazed window is more expensive than a single- and double-glazing window.

In contrast, the L-shape double-glazed window and H-shape double-glazed windows are cost-effective for the cooling-dominated (Amman and Aqaba), and continental climate (Berlin) (Jaber and Ajib, 2011) regions. The energy and cost savings potential of double and triple-glazed envelopes depends on the type of gas and thickness. An optimized double-glazed envelope saved energy by 60% in different Turkish climates (Arıcı and Karabay, 2010).

The economic incentive associated with the opaque portion of the glazed wall decreases by increasing WWR because the wall's energy savings per unit thermal resistance is reduced. Using low U and g-values’ windows in the inter medial room in multi-story office buildings reduces GHG emission nearly 4-times in the Mediterranean climate (Ávila-Delgado et al., 2021). Overall, the energy savings, cost benefits, and emission reduction depend on glazing type, area, room location, orientation, and weathering parameters (Ozel, 2019).

3.1.4. Building materials

Building envelope materials include structure, thermal insulation, thermal storage, and solar-reflective materials, as seen in Fig. 12 (Sadineni et al., 2011). Structural materials are load-bearing materials to support building structures. Traditional buildings use earthen and wooden materials with lesser mechanical strength than cementitious and geopolymer materials. Geopolymers are environmentally sustainable materials with the same thermophysical properties as cementitious materials. Insulation materials are further classified as conventional, state-of-the-art, and sustainable. Conventional insulators have medium thermal conductivity and produce more CO2 emissions in their production stage, increasing adverse environmental impacts. State-of-the-art materials have low thermal conductivity and low thermal storage, exacerbating summer overheating. Sustainable insulators have negligible environmental impacts, but they are highly flammable. Building envelope material properties significantly impact an energy-efficient and comfortable built environment design. Thermal storage materials are phase change materials. They store energy in latent form. Organic phase change materials include paraffin and fatty acids. Paraffin is widely used in building applications due to high thermal storage, super low cooling, and chemically inert structural materials, but they have high embodied energy and carbon. However, fatty acids are sustainable materials with high thermal storage, but they are disadvantageous due to their high melting point and chemically reactive behavior. The inorganic phase change materials are salt hydrates. They are low-cost, but low thermal storage, high chemical reactivity, and high supercooling temperature hinder their wider range of applications. Solar radiative materials include reflective coating, radiative cooling, and thermo-chromatic paints. They reflect solar radiation through the exterior surface of the building envelope to reduce radiative heat transfer.

3.1.4.1. Structure materials

Structure material's conductivity impacts heat transfer through the envelope, while its heat storage capacity ensures adequate thermal comfort. Structure materials absorb moisture due to inherent porosity, increasing the envelope's thermal conductivity, generating molds, and creating health hazards. Strength is a desirable material property for a load-bearing envelope (Gourav et al., 2017). Sustainable envelope materials must have low embodied energy and carbon (Hammond and Jones, 2008). The thermal conductivity of wooden and polymeric composite is the lowest amongst all other earthen and synthetic construction materials, as given in Table A2. The hemp, foamed, ultralightweight, and expanded perlite concrete has the lowest thermal conductivity (i.e., 0.09 W/m K), with poor heat storage capacity and compressive strength. However, dense concrete has high thermal conductivity, compressive strength, and moisture resistance. Environmentally, hemp, mud, and geopolymer concrete are better than other types of concrete. Ceramic and cement composite properties lies in the range of concrete structure except compressive strength of basalt fiber. On the other hand, earthen materials give better thermal and environmental performance than wooden and synthetic materials, but they have poor mechanical and hygrothermal behavior except stones which possess the highest compressive strength. Climatically, the low conductive materials and high thermal mass (ability to measure heat storage capacity) are recommended for cold and hot climate, respectively, whilst the high water vapor resistive materials are suitable for humid climates (Ungkoon and Hirunlabh, 2005). The cost-effectiveness of different structural materials is illustrated on Fig. 14.