Assessment of Green Building Materials’ Attributes to Achieve Sustainable Building Façades Using AHP

The need to enhance the performance of building façades and mitigate adverse environmental impacts has promoted the hypothesis of integrating green materials towards sustainable buildings. Façade designers tend to select building materials based on their green characteristics (origin) or green performance; however, this study highlights the importance of integrating both into the decision process. The main objective is to develop a new assessment process for selecting green building façade materials based on green performance and green originality. Furthermore, the evaluation framework considers four green building rating systems as a reference to allocate credits for the relevant criteria. Applying the proposed criteria in this study helps maximize the points for accreditation when incorporating green building materials in building façades. Moreover, the Analytic Hierarchy Process (AHP) is used to assign the proposed criteria’s weighting importance based on the four rating systems’ average points. After that, performing sensitivity analysis to identify each criterion’s influence is conducted. The study concluded that involving minimum levels of adverse impacts is the preferable criteria regarding the green origin concept. As for the green performance, promoting the performance of indoor air quality is the most favorable selection criteria.


Introduction
Building materials significantly affect the depletion of natural resources and their impact on buildings' emissions and energy usage [1]. Consequently, following the sustainability measures (environment, social, and economic) [2] and green architecture approach [3] is required to decrease the impacts of materials on the environment and building occupants. Green architecture focuses on the relationship between the building and nature, minimizing environmental impacts, health hazards, and energy and conserving natural resources [4]. In this context, the green building concept involves using eco-friendly and low-impact materials, conserving resources and energy efficiency, and optimizing indoor health for occupants [5]. The characteristics of building façades affect environmental footprint and energy performance; therefore, more awareness needs to be dedicated to their design, construction, and building materials [6].
Various researchers contribute to achieving a specific principle of green buildings to optimize the performance of the building. For instance, more solid building massing is The green origin concept means that the material is considered green based on its green characteristics (for example, it is composed of recycled or renewable resources and has minimal impact on the environment). However, the green performance concept refers to its performance over its entire life cycle, including the building's operation phase (for example, considering occupants' health and well-being and not exposing harmful substances).
The authors indicate that having a building material with green origin features does not necessarily mean that its performance is green. For instance, Gomaa et al. [20] simulated an educational building's external wall system in a hot desert climate using Autoclaved Aerated Concrete Block AAC (a green alternative material) for brickwork as a conventional material. This study mentioned that the green material positively impacts thermal comfort. Moreover, the green material slightly enhanced energy efficiency results-based on the total energy consumption-from 3245.07 to 3239.1 KWh compared to the conventional material. Another example, Phase Change Materials (PCMs), can stabilize an indoor environment for users without needing extra energy for heating or cooling [7]. However, commonly used PCMs in buildings as heat storage material have been reviewed by Chandel et al. [21] regarding their release of toxic gases that affect human health and have the potential for fires.
Therefore, the study concept adapted in this research ( Figure 1) presents the proper selection of green materials in building façade to obtain a sustainable building. The choice of green material in this study is based on its performance and its origin. Based on this concept, different evaluation criteria are proposed for obtaining a high-performance façade for sustainable buildings.

Research Scope
As mentioned in the introduction section, building materials significantly impact the environment and building users. Hence, using green materials has possible benefits related to optimizing thermal comfort, indoor health, energy efficiency, lower environmental impacts, etc. This paper aims to assess green materials for building façades according to sustainability measures and considering their green origin and green performance.
The green origin concept means that the material is considered green based on its green characteristics (for example, it is composed of recycled or renewable resources and has minimal impact on the environment). However, the green performance concept refers to its performance over its entire life cycle, including the building's operation phase (for example, considering occupants' health and well-being and not exposing harmful substances).
The authors indicate that having a building material with green origin features does not necessarily mean that its performance is green. For instance, Gomaa et al. [20] simulated an educational building's external wall system in a hot desert climate using Autoclaved Aerated Concrete Block AAC (a green alternative material) for brickwork as a conventional material. This study mentioned that the green material positively impacts thermal comfort. Moreover, the green material slightly enhanced energy efficiency resultsbased on the total energy consumption-from 3245.07 to 3239.1 KWh compared to the conventional material. Another example, Phase Change Materials (PCMs), can stabilize an indoor environment for users without needing extra energy for heating or cooling [7]. However, commonly used PCMs in buildings as heat storage material have been reviewed by Chandel et al. [21] regarding their release of toxic gases that affect human health and have the potential for fires.
Therefore, the study concept adapted in this research ( Figure 1) presents the proper selection of green materials in building façade to obtain a sustainable building. The choice of green material in this study is based on its performance and its origin. Based on this concept, different evaluation criteria are proposed for obtaining a high-performance façade for sustainable buildings.

Literature Review
Various researchers have evaluated façade building materials based on different aims and criteria. The following subsections identified the assessment criteria of green building materials and the main objectives for several studies. The authors divided the purposes of selecting green materials into three parts to clarify the importance of emerging concepts of green origin and green performance to optimize building façades. Moreover, comparing green building rating systems based on the materials-related criteria and credits helped to develop the optimal selection criteria for building façade green materials.

Literature Review
Various researchers have evaluated façade building materials based on different aims and criteria. The following subsections identified the assessment criteria of green building materials and the main objectives for several studies. The authors divided the purposes of selecting green materials into three parts to clarify the importance of emerging concepts of green origin and green performance to optimize building façades. Moreover, comparing green building rating systems based on the materials-related criteria and credits helped to develop the optimal selection criteria for building façade green materials.

Criteria for Selecting Green Building Materials
Various researchers have addressed the green material selection problem through many approaches. Table 1 illustrates the different assessment criteria of green building materials carried out by several studies. The table presents the green materials' evaluation criteria for various building elements, not just façades (for instance, selecting green flooring [22] and roofing materials [23]). Other studies aimed to identify the green building materials' selection criteria based on different purposes (for instance, considering energy efficiency [24], the building's life cycle [25], and sustainability ranking [2]). The authors highlight the sub-criteria related to the green materials' main attributes-green origin and green performance-for building façades to be used for the proposed evaluation framework (Section 5.1). Table 1. Overview of multi-criteria assessment for selecting green building materials.

Ref.
Main and Sub-Criteria [12] Four main criteria are considered:

Environmental impacts
Embodied energy/carbon emission  [10,24] Energy efficiency as a single main criterion is considered [24]: Local materials Recycled content Renewable sources Low-density industrial process Using human resources/ renewable energy Consuming less energy at the site Low thermal conductivity Criteria based on the slightest 'Greenness' requirement [10]: Mechanical properties (for structural materials) Thermal performance during the operating phase Acoustic performance Durability Weight and dimension limits Safety requirements Aesthetic Cost Impact on the environment and human health Performances connected to the specific use of buildings The color represents the green origin criteria, this color represents the green performance criterion, and this color represents the main proposed criteria.
The color represents the green origin criteria, Contribution to Energy efficiency/thermal comfort of buildings Contribution to acoustic comfort Water conservation Durability/longevity Healthfully maintained Functionality [26] Two main criteria include: Environmental criteria LCA CO2 emissions Health impact

Technological indicators
Density Specific heat Fire Classification Water vapor diffusion resistance [10,24] Energy efficiency as a single main criterion is considered [24]: Local materials Recycled content Renewable sources Low-density industrial process Using human resources/ renewable energy Consuming less energy at the site Low thermal conductivity Criteria based on the slightest 'Greenness' requirement [10]: Mechanical properties (for structural materials) Thermal performance during the operating phase Acoustic performance Durability Weight and dimension limits Safety requirements Aesthetic Cost Impact on the environment and human health Performances connected to the specific use of buildings The color represents the green origin criteria, this color represents the green performance criterion, and this color represents the main proposed criteria.
this color represents the green performance criterion, and Contribution to Energy efficiency/thermal comfort of buildings Contribution to acoustic comfort Water conservation Durability/longevity Healthfully maintained Functionality [26] Two main criteria include: Environmental criteria LCA CO2 emissions Health impact

Technological indicators
Density Specific heat Fire Classification Water vapor diffusion resistance [10,24] Energy efficiency as a single main criterion is considered [24]: Local materials Recycled content Renewable sources Low-density industrial process Using human resources/ renewable energy Consuming less energy at the site Low thermal conductivity Criteria based on the slightest 'Greenness' requirement [10]: Mechanical properties (for structural materials) Thermal performance during the operating phase Acoustic performance Durability Weight and dimension limits Safety requirements Aesthetic Cost Impact on the environment and human health Performances connected to the specific use of buildings The color represents the green origin criteria, this color represents the green performance criterion, and this color represents the main proposed criteria.
this color represents the main proposed criteria.

Selecting Façade Building Materials Based on the Green Performance
Research focused on building facades' green performance aims to minimize energy consumption and achieve indoor thermal comfort. Lin et al. [27] focused on different thickness variables of thermal mass, insulation, and other design variables in a green building located in China to consider the China Standard for Green Buildings and the building energy efficiency design standard simulating retrofit strategies. Fonseca et al. [28] simulated the addition of external EPS and XPS as insulation façade materials to a residential building aiming to evaluate energy-savings, thermal comfort, and cost savings. Huang et al. [29] added an external vacuum insulation panel in addition to other retrofitting approaches by simulating an actual residential building in China to identify energy and cost savings. Furthermore, evaluating the improvement of including 12 cm of insulation materials (EPS and rock wool) in addition to several retrofit scenarios have been made by Bellia et al. [30] in a university building in Italy to recognize energy renovation measures and applying the cost-optimum retrofit.
El-Darwish et al. [31] added EPS as an internal insulation material to a higher educational building in Egypt to minimize energy consumption. Mahdy et al. [32] evaluated three sets of external walls for low-income residential housing in three different Egyptian climatic zones to minimize energy consumption, achieve indoor thermal comfort, and guarantee maximum cost-effectiveness. Targeting to enhance the performance of building facades, some studies [6,33] assessed different wall systems in accordance with Montreal weather conditions (cold climate zone) for low-rise commercial buildings using Fuzzy measures. An optimization model for designing a sustainable building is developed by Wu et al. [34], considering energy consumption, indoor environmental quality, and life cycle cost, including exterior wall insulation and window-to-wall area ratio.

Selecting Façade Building Materials Based on the Green Origin
Studies have been conducted targeting the green origin of building materials to mitigate the environmental impacts using life cycle assessment (LCA). Prateep et al. [9] proposed using six different wall construction schemes in tropical climate zones to identify the environmental impacts and the social assessment. Ben-Alon et al. [35] suggested utilizing an alternative wall construction (cob earthen) for conventional materials (concrete masonry and wood wall) and identified cob construction's environmental performance from a life cycle perspective. Assessing different wall systems based on their Global Warming Potential and Fossil Fuel Consumption using LCA has been studied by Dekkiche et al. [36] in a certified LEED gold building. Vilcekova et al. [13] analyzed different building materials for exterior walls in terms of embodied energy and embodied carbon.

Selecting Façade Building Materials Based on Performance and Origin
The selection of façade materials that meet the performance and green properties requirements has been evaluated in other studies. Farahzadi et al. [37] compared using conventional and alternative environmentally friendly building materials regarding energy consumption and carbon dioxide production in a typical residential building. Optimizing building energy consumption and environmental impacts have been studied [38,39] using LCA, simulation, and building information modeling (BIM). Balali et al. [14] identified six smart materials' weightings and relative importance for façades based on technical, economic, social, environmental, and energy consumption criteria using SPSS software and AHP method to select the most sustainable smart material for building façades. Moussavi et al. [12] classified the most sustainable alternative between five façade material systems based on AHP regarding environmental impacts, life cycle cost, performance, and social benefits. Mostavi et al. [40] identified the optimum design based on the life cycle cost, environmental impacts, and occupant satisfaction considering a database of 65 different materials in an office building. Leo et al. [41] analyzed eight buildings' thermal performance constructed with eco-friendly, durable, and cost-effective structures regarding surface and indoor air temperatures and thermal comfort. Gomaa et al. [20] identified the energy con-sumption, embodied carbon, and thermal comfort when replacing a conventional material with an environmentally friendly one for an external wall using simulations.
Although the reviewed studies could enhance the design of sustainable building facades using better green building materials, limitations lessen their application effectiveness. Firstly, considering selecting façade materials based on green properties, researchers mainly focus on utilizing LCA. Secondly, researchers rely primarily on the performance of façade selected materials in terms of energy efficiency. On the other hand, green performance and origin of building materials have been considered based on limited disciplines. Moreover, various previous relevant optimization approaches do not reflect the whole picture. Thus, incorporating multiple disciplines' performance and origin assessment attributes is essential to accomplishing a green building envelope.

Comparison of Green Building Rating Systems
A comparison is carried out between four green building rating systems (LEED [42], BREEAM [43], GPRS [44], and Estidama [45]) to identify the highest consideration of categories' credits. Considering the total points, the first consideration for LEED, BREEAM, and Estidama is the Energy category with 30%, 21%, and 25%, respectively. Moreover, GPRS grants the highest weightings for Water Efficiency with 30% then Energy Efficiency with 25%. As for the materials' category, LEED awards 12% approximately of the total weightings in the third level of interest, and the fourth level for BREEAM, GPRS, and Estidama with 9%, 10%, 16%, respectively.
Materials' related categories in the four green rating systems' technical manuals have been studied to determine the sub-criteria for selecting green building façade materials. Moreover, identifying the possible credits for each sub-criteria compatible with façades to derive the relative weights. Table 2 illustrates the proposed matched sub-criteria and credits of green building façade materials derived from the four green rating systems' manuals. It can be noticed that green building materials' main attributes (green performance and green origin) comprise various categories (the proposed evaluation criteria of green materials will be identified later in Section 5.1).
Green origin criterion of façade building materials is mainly comprised of the materials' category (namely, for LEED: "Materials and resources", for BREEAM: "Materials", for GPRS: "Materials and resources", and for Estidama: "Stewarding Materials").
However, the green performance criterion is comprised mostly of the categories of indoor environmental quality and energy categories (namely, for LEED: "Indoor Environmental Quality", "Energy and Atmosphere", for BREEAM: "Health and Wellbeing", "Energy", for GPRS: "Indoor Environmental Quality", "Energy Efficiency", and for Estidama: "Livable Indoors", "Resourceful Energy"). * refers to the proposed evaluation criteria, which will be discussed in Table 3.

Methodology
In developing systematic decision criteria, Figure 2 illustrates the steps involved in the research. Firstly, a literature review (Section 3) on the assessment criteria for selecting green building materials was discussed by previous studies. Additionally, identifying the criteria relevant to the concepts of green origin and performance of materials for the green building rating systems. The research is based on determining and assessing the criteria for selecting green building façade materials. Since there is no academic standardization procedure to measure these criteria, this paper has been linked to green certification protocols as a reference guide for measuring their importance. Furthermore, green building certification systems encourage using environmentally friendly materials in sustainable projects [46].

Methodology
In developing systematic decision criteria, Figure 2 illustrates the steps involved in the research. Firstly, a literature review (Section 3) on the assessment criteria for selecting green building materials was discussed by previous studies. Additionally, identifying the criteria relevant to the concepts of green origin and performance of materials for the green building rating systems. The research is based on determining and assessing the criteria for selecting green building façade materials. Since there is no academic standardization procedure to measure these criteria, this paper has been linked to green certification protocols as a reference guide for measuring their importance. Furthermore, green building certification systems encourage using environmentally friendly materials in sustainable projects [46]   The next step is developing the evaluation criteria for green building façade materials that merge green origin and performance concepts to fulfill sustainability measures (Environment, Social, and Economic). Moreover, allocating scores to the proposed criteria is achieved by considering the materials' credits of the green rating systems as a reference. Additionally, a comparison is carried out to identify the difference between the original materials' credits and the proposed material evaluation for obtaining green building rating systems accreditation. Then, AHP is performed using super decision software to determine the highest and the lowest priority evaluation criteria. Finally, final weightings are compared, which assists decision-making in selecting the proper green material for a sustainable building façade.

Green Building Materials' Evaluation Criteria
Based on the study of materials-related categories in four green building rating systems, in addition to results obtained from previous studies, five main criteria can be defined to assess both the origin and performance of façade green building materials ( Table 3).
The main criteria have been selected based on sustainability assessment measures (Environment, Social, and Economic). The authors also believe that selecting the optimum green material for façades requires the material to be resource-efficient and energy-efficient. This assumption is based on utilizing green materials to reduce energy consumption and conserve natural resources. Therefore, the green material depends on different variables to be a resource-efficient material. As to accomplish the goal of energy conservation, energysavings are considered either during the material's life cycle or the operational energy (due to the different heat transfer of each façade material), which affects the greenness aim. Table 3. Proposed assessment criteria for green building materials and their description.

Main Criteria Green Materials' Attributes Sub-Criteria Description Unit
Green origin (O) The goal is to reduce material use, waste, and energy consumption by selecting regional, recycled, reusable, renewable, and durable materials, promoting resource-efficient building materials.
--Life expectancy (P1a) The selected building materials are expected to have a long-life expectancy as it affects the façade systems' serviceability by reducing exposed materials' short time until degradation and minimizing maintenance costs.
years -Weather, moisture, fire-resistant (P1b) Selecting green materials capable of complying with climatic conditions to be weather, moisture, and fire-resistant is favorable. -

1-Resource efficiency
Green performance (P) -Compatibility with climate zone (P1c) The goal is to select green materials compatible with climatic regions (as one of the buildings' passive design measures), affecting thermal characteristics and material thickness.
--Involve minimum levels of negative impacts (O2a) The aim is to have lower environmental impacts of the built environment that can be performed using life cycle assessment LCA from cradle to cradle. It reflects the performance of green building materials after installation during the operation stage to identify indoor air quality performance.
--Efficient waste disposal (P2b) The target is to reduce construction and demolition waste to minimize the environmental impacts of construction, demolition, and diverted construction waste to material reuse or recycle.

Weight or volume 2-Environmental impacts
Green performance (P) -Healthfully maintained (P2c) The goal is to select materials that do not release any toxic contents in the maintenance process. - Table 3. Cont.

Main Criteria Green Materials' Attributes Sub-Criteria Description Unit
-Aesthetics (O3a) The aesthetic criterion is relative from one person to another, depending on the designer's preference based on the quality of the material finish [12].

Points
Green origin (O) -Certified materials (O3b) The material with a verified document (Environmental Product Declarations) makes it easier to select the material with less environmental impact within sustainable buildings [47]. Thus, it increases the awareness of using documented materials [48].
--Thermal performance (thermal resistance) (P3a) Identifying the good thermal resistance is based on the type of materials, the thickness that affects heating and cooling load requirements. Thus, thermal resistance contributes to occupants' thermal comfort. Selecting proper building materials affects having acceptable thermal comfort levels, which is an essential building characteristic. Additionally, it affects energy consumption levels [49]. The satisfaction of buildings' occupants regarding health and comfort has a social sustainability impact [50,51].

Index -Effect on Acoustics (P3c)
Reducing indoor noise can be effective using proper external building materials [52], affecting occupants' satisfaction levels.
The average noise level (Leq)

3-Social impacts
Green performance (P) -Ease of implementation (P3d) It reflects the selected green materials to be easy to handle and have the required expertise and skill for labor for installation.
--Affordability from cradle to cradle-Life Cycle Cost (O4a) The goal is to optimize the cost-effectiveness of alternative building materials from cradle to cradle, including reasonable capital, transport, operation, labor, installation, maintenance, and replacement cost [53].

Monetary units based on the currency in a country Green origin (O) -Low Embodied Energy cost (O4b)
It reflects the cost of energy utilized during all building materials' processes.

4-Economic impacts
Green performance (P) -Optimize the use of materials during the design (P4a) The goal is to optimize materials in building design utilizing lightweight materials and materials with standard dimensions and designing for material reuse. Achieving material usage reduction while designing optimization procedures has an economic impact (economic profit) [46].

Monetary units Green origin (O)
-Energy efficiency during their life cycle (Embodied Energy) (O5a) Low energy is demanded during the production processes of building material on-site construction, demolition, and disposal [14].

5-Energy efficiency
Green performance (P) -Effect on energy consumption (P5a) The impact of external envelope materials on the operating energy through heating and cooling affects the design of energy-efficient building facades.

Attributing Scores to Green Materials' Assessment Criteria
To facilitate comparisons of the evaluation criteria, attributing scores to each subcriteria of the proposed framework is based on the equivalent credits presented in green building rating systems' manuals, as depicted in Table 2. Figure 3 represents the credits of sub-criteria conducted by green rating systems and the proposed average credits. The proposed assessment credits are calculated based on the average credit of the total four green rating systems' points (which will be used later in Section 5.4) to represent decisionmakers' judgments.
To facilitate comparisons of the evaluation criteria, attributing scores to each subcriteria of the proposed framework is based on the equivalent credits presented in green building rating systems' manuals, as depicted in Table 2. Figure 3 represents the credits of sub-criteria conducted by green rating systems and the proposed average credits. The proposed assessment credits are calculated based on the average credit of the total four green rating systems' points (which will be used later in Section 5.4) to represent decisionmakers' judgments. The highest average credit points among all assessed sub-criteria in four green building rating systems are for the effect on energy consumption and thermal performance, as shown in Figure 3, followed by involving minimum levels of adverse impacts. The same distribution is parallel to LEED and Estidama credits; however, the second-highest order for Estidama is for the sub-criteria of "recycled content". As for BREEAM, the highest credit is for involving minimum levels of adverse impacts. Moreover, GPRS awards the highest credit for thermal performance.
The selected green rating systems do not directly cover five sub-criteria credits; therefore, they are assumed based on the most relevant sub-criteria credit. The assumed subcriteria can be derived as follows; (1) 'Weather, moisture, fire-resistant' (P1b) is estimated to equal the credit point of 'Compatibility with climate zone' (P1c). This estimation is based on the green rating systems methodology, which award points to projects integrating passive design measures. One of these measures is that the building has to be adapted to climate change. Thus, sub-criteria credits of P1b and P1c are equals. The basis of this assumption is the methodology that there is a relationship between embodied energy and carbon footprint [54].
Furthermore, the assessment methodology of energy efficiency in the green rating systems is based on the percentage of energy consumption reduction in the proposed building. Otherwise, LEED analyzes the efficiency measures in terms of energy cost and CO 2 emissions. Various studies identified the different percentages of optimized energy consumption using alternative building materials for external walls. For example, Zuhaib et al. [55] achieved a 65% reduction of final energy consumption using scenarios of envelope systems and materials in low-energy residential buildings compared to a reference case. Another study by Fonseca et al. [28] achieved 39% energy savings using active and passive building refurbishment measures. A further reduction in energy consumption was achieved by El-Darwish et al. [31], of 33%, using some of the building's envelope features for retrofitting. Therefore, aiming to have a standard assessment measure for all green building rating systems, the sub-criteria of the effect on Energy consumption (P5a) is calculated based on an average reduction of 30%.

Analysis Results for Green Materials' Credits of Green Building Rating Systems
Analyzing the selected green building rating systems' materials aspects based on the proposed assessment criteria (Tables 2 and 3) is concluded in Table 4. Therefore, Table 4 compares the total credits of green materials' main attributes (green origin and performance) among the four rating systems. Moreover, it represents each rating system's original total points for the materials' category-without adding assumption points-and the total final points for all categories. As shown in Table 4, higher credits are given to green origins for LEED certification than green performance. However, GPRS and Estidama award more green performance points than green origin, and BREEAM awards the same credits for both main attributes. Additionally, selecting green materials for building façades-regarding the proposed evaluation criteria points-helps decision-making obtain more points for accreditation than the original points of materials' category. This maximization is based on the selection of green materials that combine their green origin and performance criteria. More than double percentage weightings are achieved for the proposed criteria's total percentage compared with the original percentage of the materials' category.
Analyzing the preferable sub-criteria among green rating systems-regarding the proposed framework's highest points (Table 2)-has revealed that the green origin concept is identified from the sub-criteria involving minimum levels of adverse impacts for LEED and BREEAM. Moreover, GPRS mainly focuses on low-emitting materials. Furthermore, Estidama awards the highest points to the sub-criteria of recycled content. As for the green performance concept, LEED and Estidama award the highest credit to the effect on energy consumption sub-criteria. Otherwise, BREEAM awards more points similarly to the sub-criteria of promoting indoor air quality performance and the effect on acoustics. As for GPRS, the sub-criterion of thermal performance has the highest credit.

Formulation of the Proposed Criteria Using AHP
Super Decisions software v.3.2 [56] is used to solve the decision-making problem based on AHP. The steps of a typical AHP procedure are ranged as follows [57]; 1. Developing a hierarchical model: The hierarchy consists of clusters arranged in levels to analyze the decision. The first level is the primary goal connected to five main criteria. Each criterion's cluster has a group of sub-criteria which are connected to two alternatives (attributes): green origin (O) and green performance (P) (Figure 4). 2. Deriving relative weights for the criteria: In this step, a pairwise comparison matrix is made for each criterion based on the primary goal.
consumption sub-criteria. Otherwise, BREEAM awards more points similarly to the subcriteria of promoting indoor air quality performance and the effect on acoustics. As for GPRS, the sub-criterion of thermal performance has the highest credit.

Formulation of the Proposed Criteria Using AHP
Super Decisions software v.3.2 [56] is used to solve the decision-making problem based on AHP. The steps of a typical AHP procedure are ranged as follows; [57] 1. Developing a hierarchical model: The hierarchy consists of clusters arranged in levels to analyze the decision. The first level is the primary goal connected to five main criteria. Each criterion's cluster has a group of sub-criteria which are connected to two alternatives (attributes): green origin (O) and green performance (P) (Figure 4). 2. Deriving relative weights for the criteria: In this step, a pairwise comparison matrix is made for each criterion based on the primary goal. Therefore, weights are calculated based on the values of the proposed credits. Table 5 represents each criterion's proposed points using the average credits of four standard green rating systems as a reference with the assumption of five sub-criteria (mentioned in Section 5.2). AHP ranks the fundamental evaluation scale from 1 to 9 (Table 6) [57], where 1 represents equal or weakly more important and 9 illustrates extremely important (was developed initially by Saaty [58]). * refers to assumed credit, a refers to the total points for green origin and performance, and b refers to the main criteria' total points.
The judgment matrix originated based on each pair's relative importance in each cluster, including the main criteria and sub-criteria involved in the decision (see Appendix A for the pairwise comparison of the main criteria and Appendix B for the judgment matrix for social impacts' sub-criteria). For example (considering the social impacts), the subcriteria credit point P3b equals P3c, which means the importance ratio is 1. Additionally, P3a has the highest point-between sub-criteria of the social impacts-than other sub-criteria, while O3a, O3b, and P3d have the lowest points. Thus, the importance ratio equals 9, which is assigned as an input for this comparison. After entering the values of the importance ratio for each main and sub-criteria pair, the inconsistency rate is checked by the software. It is considered to be less than 10%, which validates that the assumptions are acceptable. Therefore, the inconsistency index of each pair of main criteria (10 comparisons) is 0.04219. After that, each sub-criteria pair is compared based on the main criteria (68 comparisons). Therefore, the inconsistency ratioderived from the software-ranged between 0.01387 for resource-efficiency sub-criteria, 0.04033 for environmental impacts sub-criteria, 0.01952 for social impacts sub-criteria, and zero for economic impacts and energy efficiency.
3. Deriving local priorities for the alternatives: It is calculated based on the importance comparison between each alternative (green origin and green performance) regarding each sub-criteria (26 sub-criteria). Appendix C represents an example of the local priorities based on the sub-criteria O3a. As shown in this figure and based on the points in Table 5, O3a has the lowest point between sub-criteria, and the total points for green performance (for the main criteria social impacts) have the highest value. Therefore, green performance is ranked 9 as extremely more important than green origin regarding the sub-criteria O3a. 4. Deriving overall priorities (model synthesis): The weights are combined to synthesize the ratings and calculate the overall economic impact priorities. Therefore, the final ranking criteria of green building materials indicate three different levels (Table 7) as follows: a. Ranking of main criteria (represents the priority of each main criteria): Analyzing the main criteria's weights revealed that the environmental impacts criterion has the highest weight among main criteria with 0.47. However, both the economic impacts and energy efficiency criteria have the lowest weights with 0.04. As expected, the economic impacts-as the main criteria-has the least weight of priority. For instance, Illankoon et al. [59] mentioned a lack of evaluation for most green building rating systems' economic aspects.
b. Ranking of local criteria (represents the priority of sub-criteria based on main criteria): Resource efficiency (with eight sub-criteria): The preferable sub-criterion for green origin is recycled content, similarly to the weighting of the preferable sub-criteria for green performance weather, moisture, fire-resistant with 0.30.
Environmental impacts (with seven sub-criteria): The preferable sub-criterion for green origin involves minimum levels of adverse impacts, and the preferable sub-criterion for green performance is promoting the performance of Indoor Air Quality with 0.41, 0.23, respectively.  Social impacts (with six sub-criteria): Both sub-criteria for green origin aesthetics and certified materials are equal to 0.05. Moreover, the preferable sub-criterion for green performance is the thermal performance with 0.60.
Economic impacts (with three sub-criteria): Both sub-criteria for green origin Life Cycle Cost and low Embodied Energy cost are equal with 0.44, while the sub-criterion for green performance optimizing the use of materials during the design has 0.11.
Energy efficiency (with two sub-criteria): The green performance sub-criterion affecting the energy consumption has a significantly higher weight than the green origin sub-criterion of energy efficiency during their life cycle, with 0.90 and 0.10, respectively. c. Ranking of global criteria (represents the final priority of each sub-criteria) Considering the global weight of sub-criteria, involving minimum levels of adverse impacts is the highest importance sub-criterion based on the final green origin scores with 0.193. Additionally, promoting Indoor Air Quality's performance has the highest weight, with 0.110, influencing green performance.
Moreover, there is no significant difference between the final weightings-obtained from the software-of green origin and green performance with an overall score of 0.53 and 0.47, respectively. This result is evident due to the close of total points among green rating systems (Table 4). 5. Perform sensitivity analysis: (will be discussed later in Section 5.5). 6. Making the final decision: The results representing the highest and the lowest priority evaluation criteria of green buildings, decision-makers could identify the proper green material for each project.

Sensitivity Analysis
Performing sensitivity analysis is essential for identifying which criteria motivated the initial results and the robust final decision [47].
Regarding the overall ranking (Table 7), Figure 5a illustrates that the green performance of green building materials is influenced by the main criteria of social impacts and energy efficiency more than other criteria. In contrast, the green origin is motivated more by economic impacts. As for environmental impacts and resource efficiency, they are relatively close concerning the green origin criterion. Moreover, considering global weight from Figure 5b, environmental impacts criteria significantly affect green origin and performance. After that, resource efficiency has a significant influence on both main attributes. Moreover, the green performance of social impacts is close to resource efficiency. Super decision software could identify the sensitivity of results by specifying the criteria effect if the weights are different ( Figure 6). There is a minor difference between the total of green origin and green performance; thus, determining each criterion's effect is essential. From Figure 6c,e, social impacts and energy efficiency significantly influence green performance than green origin. As for Environmental impacts (Figure 6b), there is almost no difference between both main attributes. Moreover, resource efficiency ( Figure  6a) and economic impacts (Figure 6d) have more influence on green origin than green performance. Super decision software could identify the sensitivity of results by specifying the criteria effect if the weights are different ( Figure 6). There is a minor difference between the total of green origin and green performance; thus, determining each criterion's effect is essential. From Figure 6c,e, social impacts and energy efficiency significantly influence green performance than green origin. As for Environmental impacts (Figure 6b), there is almost no difference between both main attributes. Moreover, resource efficiency ( Figure 6a) and economic impacts (Figure 6d) have more influence on green origin than green performance.

Comparing the Previous Studies' Results
In this section, a comparison of the obtained findings with the relevant previous studies outcomes has been made to analyze the most important criteria for evaluating green building materials. Various studies represent different criteria based on the top ranking of green materials selection. Aiming to determine the most sustainable system for building façades, Moussavi et al. [12] recognized embodied energy and carbon emission, followed by material costs, as the top criteria. Khoshnava et al. [2] considered the affordability of green building materials as the top ranking. In contrast, resource efficiency focused on natural, renewable materials is the second top criterion based on sustainability measures (environment, social, and economic). Super decision software could identify the sensitivity of results by specifying the criteria effect if the weights are different ( Figure 6). There is a minor difference between the total of green origin and green performance; thus, determining each criterion's effect is essential. From Figure 6c,e, social impacts and energy efficiency significantly influence green performance than green origin. As for Environmental impacts (Figure 6b), there is almost no difference between both main attributes. Moreover, resource efficiency ( Figure  6a) and economic impacts (Figure 6d) have more influence on green origin than green performance.

Comparing the Previous Studies' Results
In this section, a comparison of the obtained findings with the relevant previous studies outcomes has been made to analyze the most important criteria for evaluating green building materials. Various studies represent different criteria based on the top ranking of green materials selection. Aiming to determine the most sustainable system for building façades, Moussavi et al. [12] recognized embodied energy and carbon emission, followed by material costs, as the top criteria. Khoshnava et al. [2] considered the affordability of green building materials as the top ranking. In contrast, resource efficiency focused Moreover, decreasing energy consumption is the most remarkable selection criteria, according to Balali et al. [14], aiming to identify the most sustainable building façades' smart material. Furthermore, the availability and cost of green materials were highlighted as the most important criteria for evaluating green materials based on LEED, according to Castro-Lacouture et al. [60]. The environmental impacts, economic and the R-value are equally crucial as assessment criteria for selecting the optimum external building wall system, according to Prateep et al. [9].
As for this study's findings, environmental impacts involving minimum levels of adverse impacts and promoting Indoor Air Quality performance are the most critical criteria that merge green origin and performance, considering four green rating systems as a reference. The second top ranking is resource efficiency with recycled content and weather, moisture, and fire-resistant sub-criteria. The research recognized 14 sub-criteria for the green origin and 12 sub-criteria for green performance, distributed to five main criteria. These criteria are based on achieving sustainability measures to select the proper green material for building façades.
In the authors' opinion, the proposed framework weights of importance could be adapted based on each case study's requirement (climate zone, building type, etc.). Hence, it could require certain assumptions to determine the highest priority criterion that would be the basis of the selection process.

Conclusions
This study identifies the assessment criteria incorporating green origin and performance of green building façade materials based on sustainability aspects. Therefore, a literature review of the green material selection approaches is conducted to optimize the building façades' materials selection process. The researchers lean towards assessing green façade materials based on limited disciplines that reflect green performance and green origin concepts of façade materials. The authors highlight the importance of selecting the proper green materials for façades that require merging green origin and green performance, considering green building rating systems as a reference.
Therefore, five main criteria and 26 sub-criteria are identified throughout the study, which could be valuable for selecting building façade material for accreditation purposes or following sustainability standards in new or refurbished buildings. Four green rating systems were used to determine the relevant criteria and credits of using green materials to have more realistic numbers based on experts' opinions. Applying the proposed evaluation criteria for selecting green materials for building façades helps earn more credits for green rating systems' accreditation than the original credits for materials. Therefore, more than double the percentage of total credits is achieved for the analyzed green rating systems based on the proposed criteria. The green certification protocols are varied towards focusing more on earning points for green origin or performance criteria. The highest importance is given to green origin criteria than green performance for LEED; however, earning points for both main attributes in BREEAM are the same. As for GPRS and Estidama, more points are awarded to green performance than the green origin.
Furthermore, averaging each sub-criteria score of the four rating systems to develop the proposed evaluation criteria credits. Moreover, the average scores are used for criteria prioritization using the AHP process. It is preferable to select materials for façade buildings based on their green origin and performance, in the authors' opinion based on the results of AHP. However, for decision-makers who cannot consider the whole criteria for new or renovated buildings, the most priority criteria can be selected based on the final scores of the AHP method. It can be concluded that involving minimum levels of adverse impacts is the preferable selection sub-criteria for green origin. Promoting indoor air quality performance is the most favorable selection for green performance with 19%, 11%, respectively, for global weight. Regarding sensitivity analysis results derived from the software, social impacts and energy efficiency significantly influence green performance than green origin. Moreover, resource efficiency and economic impacts have more influence on green origin than green performance.
There are possible limitations of using AHP in determining the relationships among criteria and sub-criteria (especially for evaluating alternative green building materials in a case study). The decision is complex since several criteria are not based on numerical analysis. Therefore, other techniques (e.g., ANP) could be adopted in future work. Additionally, this study's findings can be further extended by evaluating the green origin and the green performance of different green materials and assessing their effect on the performance of the whole building. Moreover, they could assist in developing criteria for selecting green materials for other building elements (i.e., flooring and roofs) that comply with merging between green origin and performance.  Appendix A Figure A1. A sample of the pairwise comparison matrix for the main criteria. Figure A1. A sample of the pairwise comparison matrix for the main criteria.

Appendix B
Buildings 2021, 11, x FOR PEER REVIEW 26 of 29 Appendix B Figure A2. A sample of the pairwise comparison matrix for social impacts sub-criteria.
Appendix C Figure A3. A sample of local preferences for each alternative with respect to sub-criteria.