Design strategies using multi-criteria decision-making tools to enhance the performance of building façades

Journal Pre-proof Design strategies using multi-criteria decision-making tools to enhance the performance of building façades Moghtadernejad Saviz, Chouinard Luc E, Mirza M. Saeed PII:

S2352-7102(19)31959-X

DOI:

https://doi.org/10.1016/j.jobe.2020.101274

Reference:

JOBE 101274

To appear in:

Journal of Building Engineering

Received Date: 27 October 2019 Revised Date:

11 February 2020

Accepted Date: 12 February 2020

Please cite this article as: M. Saviz, C. Luc E, M.M. Saeed, Design strategies using multi-criteria decision-making tools to enhance the performance of building façades, Journal of Building Engineering (2020), doi: https://doi.org/10.1016/j.jobe.2020.101274. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Author Statement Saviz Moghtadernejad: Conceptualization, Methodology, Software, Investigation, Writing Original Draft, Visualization; Luc Chouinard: Writing - Review & Editing, Supervision, Funding acquisition; Saeed Mirza: Writing - Review & Editing, Supervision, Funding acquisition

Design strategies using multi-criteria decision-making tools to enhance the performance of building façades Moghtadernejad, Saviz 1,4 ; Chouinard, Luc. E.2 ; and Mirza, M. Saeed 3 1,2,3 McGill University, Canada 4 [email protected]. Abstract The climate change crisis and the need to control environmental impacts of the construction industry have motivated designers toward designing high performance and sustainable buildings. A building’s façade, as a part of the enclosure, is certainly no exception to this requirement; however, despite the significant potential for building energy savings and reduction of environmental impacts, building façade design is not receiving proper attention. Presently, there is a need for a systematic approach to facilitate the integration of the various disciplines involved (e.g. architecture, as well as structural, mechanical and electrical design) and provide a comprehensive action plan that considers the life cycle stages of a façade system from conceptual design to demolition. This paper initially provides the required actions and considerations in each phase of design to provide a new and simplified guideline for designers in achieving a high performance façade system, with the help of Multi-Criteria Decision-Making (MCDM) methods. Secondly, the application of the Choquet integral and the Analytic Hierarchy Process (AHP) is discussed to select the most suitable alternatives in façade preliminary design for a case study building, and the results are compared. Keywords: High-performance façade design, Systematic façade design, Sustainable façades, Optimal façade design, Decision-making in façade design 1 Introduction In recent years, designing high-performance building façades has received increasing attention. There has been extensive research done on the optimization of building performance with respect to one or more façade design criteria. However, these studies are mainly focused on passive design strategies [1, 2], or improving occupant-centric metrics such as optimizing the energy performance of the building [3, 4] or the daylighting [5, 6] and glare control [7]. Although addressing occupantcentric issues are very important, a comprehensive assessment of facade performance yields a far more complex set of considerations that are lacking in current design practises, up to the point that Patterson and Matusova [8] claim that the newly built facades are often incorrectly labeled as high-performance. As facades integrate multiple and diverse systems, the design process intrinsically involves the participation of architects and civil, mechanical and electrical engineers, and design from one domain of expertise can affect the performance of other designs from other domains. The inherent complexity of the design process makes it difficult for designers from a given discipline to consider all the other functions of the system. Consequently, although an interactive and integrated design approach is desirable for such systems; most façade designers still tend to use traditional sequential design approaches [9]. In traditional façade design practice, a specialty contractor working as a member of the construction team, performs the design, manufacturing and assembly of façade systems [9]. In each project, the selection of a façade alternative is the most important step in designing an optimal façade system. This task is undertaken at the architectural design or the preliminary design stage, where the designer defines the performance requirements of the system with respect to the project needs and the provisions of the codes and standards. National codes and standards in each country have some provisions and minimum requirements for designing building envelopes in terms of structural integrity, occupant-centric criteria and energy efficiency. Subsequently, the designer selects a few alternatives that satisfy the performance requirements based on personal experience and selects an alternative in consultation with the clients [10]. The problem with this approach is that although the method works well when the goal is to satisfy the minimum requirements of the project and the codes and standards, the design rarely results in an optimal solution. Another issue with design practices that only satisfy the requirements of the codes and standards is that they do not adequately address the issues of resilience, environmental impacts and sustainability, inspection needs and maintainability. Moreover, current design practices are rarely influenced by the results of the recent studies on improving the building façade performance. This is mainly

because the researchers have tackled the current issues separately and have not provided a comprehensive guideline and a systematic approach to provide a balance among design criteria from various disciplines. Multi-criteria decision-making (MCDM) methods are appropriate tools for providing the required balance between design objectives. There are several MCDM methods available to the designers, which utilize single or hybrid approaches. The advantages and limitations of most-commonly used MCDM methods in civil engineering are summarized in [11-15]. However, application of multi-criteria optimisation and decision support tools in façade design procedures is very limited and recent. Table 1 categorises and provides a brief overview of the literature on the optimization of building performance with respect to one or more façade design criteria, along with studies using MCDM methods in façade preliminary design stage. Studies suggest that the Analytic Hierarchy Process (AHP) is the preferred method for dealing with multi-criteria decisionmakings in sustainable design [16]. TOPSIS (Technique for Order Preference by Similarity to Ideal Solutions), VIKOR (VIekriterijumsko KOmpromisno Rangiranje) and ELECTRE (Elimination and Choice Translating Reality) are among other commonly-used methods [14, 17]. However, all these methods have limitations when design criteria interact. A method that overcomes the limitations of current MCDM methods is the Choquet integral [18] since this method can explicitly account for the interactions among criteria. This method can be integrated with AHP to assign preferences among design criteria. The objective of the paper is to provide a systemic and efficient approach for the design of building facades. This is achieved by providing a comprehensive guideline for each design phase of a high-performance building façade. The guideline integrates the findings of previous studies for improving the performance of facades using passive and active strategies and uses the Choquet integral and AHP in preliminary façade design phase to account simultaneously for multiple design criteria. The proposed process is demonstrated for a case study with four façade alternatives and 15 design criteria and the results are compared.

Table 1. Review of recent literature on improving the performance of building facades.

Application of decision support tools in façade design

Improving Occupant-centric metrics

Area of focus

References

Building envelopes and energy efficiency in buildings with a life cycle approach

[3, 4]

Energy performance of Photovoltaic (PV) modules as adaptive building shading systems

[18]

The utilisation of a dynamic Building Integrated Photovoltaic (BIPV) system for adaptive shading to improve building energy performance by controlling solar heat gains and natural lighting, while simultaneously generating electricity on site.

[19]

Design and control optimisation of adaptive insulation systems for office buildings

[20, 21]

Solar shading in low energy office buildings

[5]

Optimisation criteria of an active transparent façade based on IEQ and energy efficiency issues

[22]

Challenges and trends in design optimisation of solar shading systems for tropical office buildings

[6]

The influence of shading control strategies on the visual comfort and energy demand of office buildings

[23]

Application of Genetic Algorithms (GA) to facilitates the exploration of facade designs based on illuminance and/or glare objectives

[7]

Multi-criteria assessment of facade alternatives using WSM (Weighted Sum Model), WPM (Weighted Product Model) and joint method of the latters called WASPAS (Weighted Aggregated Sum Product Assessment)

[24]

Multi-criteria selection of façade systems based on sustainability criteria

[25]

Application of Multi-criteria decision-making methods in preliminary design stage of sustainable facades

[26]

Implementation of a multi-criteria and performance-based procurement procedure for energy retrofitting of facades during early design

[27]

A framework for generating and evaluating façade designs using a multi-agent system approach

[28]

A novel method for improving building facade performance, based on multi-objective optimization using the genetic algorithms in combination with building performance simulations; taking into consideration

[29]

Performance assessment and evaluation

occupant comfort, energy consumption and energy costs. Building Performance Simulation and characterisation of adaptive facades

[30]

Design for façade adaptability–towards a unified and systematic characterization

[31]

Current trends and future challenges in the performance assessment of adaptive façade systems

[32]

Thermal and electrical performance of an integrated PV-PCM system in double skin façades

[33]

Review of current status, requirements and opportunities for building performance simulation of adaptive facades

[34]

Simulation-based evaluation of adaptive materials for improved building performance

[35]

A simulation framework for the evaluation of next generation responsive building envelope technologies

[36]

2 Methodology The building codes and standards in each country provide minimum requirements for designing environmental separators including building envelopes. However, the design procedure should not be only about selecting a design alternative that meets the performance requirements of the codes and standards and project expectations. The goal is to select an alternative that maximizes all performance criteria. Since some criteria are conflicting with each other, e.g. initial costs and energy efficiency, it is not feasible to maximize all criteria. Hence, there is a need for providing a balance among criteria so that the overall performance would be maximized. Presently there is no guideline to design an optimal façade system that considers all necessary criteria from various involved disciplines. In developing a systematic decision-making process the designer must first define project goals, limitations, and constraints [26]. Generally, the project goals cannot be attained in a single step and a sequence of multiple actions is required to achieve the optimal solution, otherwise, the design may evolve through unnecessary, time consuming and costly iterations [37]. The sequence of actions and considerations for façade design are divided into three stages, namely, early building design, façade preliminary design, and façade detailed design. Figure 1. Presents an overview of the proposed conceptual framework for designing building facades. Moreover, there are also some provisions that must be accounted for during the construction and operation phases to ensure the designed systems will perform as expected. These stages are discussed in the following sections.

Figure 1. Conceptual framework for designing building façades

2.1 Considerations during building design stage Building enclosures are designed to protect the occupants from cold, heat, precipitations, solar radiation, outside noise and pollution, prevent fire spread, be structurally safe, durable, cost-efficient and aesthetically pleasing [38]. However, before beginning the building design procedure, the designer must decide on the importance of each of these attributes since design priorities, will influence the decisions throughout the whole design procedure. Each project has limitations and constraints. Some limitations may be associated with site characteristics (such as shape, size, and slope) or the available budget. Other limitations, such as municipal regulations may restrict the choices in materials for the building façade. To avoid any unnecessary iterations, it is best to identify the constraints at the earliest stage of building design and adopt appropriate strategies to obtain the best possible solutions despite the constraints. 2.1.1

Building massing and orientation

Building massing and orientation directly influence the fenestration layout and have an important impact on building performance and operating costs and when incorporated at the earliest stages of building design, will eliminate the need for some corrective strategies, such as automated shading and operable windows which are more complex and costly [39]. It is recommendable to minimize façade exposure on the east and west elevations and orient the building so that its long elevations face north and south. This facilitates the control of solar heat gain through the implementation of exterior shading. It is also necessary to consider an optimum floor plate depth along with the layout of core spaces and services. This will enhance the effective distribution of daylight in building spaces. For example, a floor depth greater than approximately 12 meters will lead to the formation of an internal zone with very limited access to the façade. This will result in the need for a mechanical ventilation system. In mild climates minimizing the floor plate depth in combination with a properly shaded façade system (and sometimes application of operable windows) can result in the elimination of the need for air conditioning systems which is a more energy efficient solution [39]. However, in some cases due to site constraints, it may not be possible to achieve optimal building orientation. Hence, the design team should seek other strategies, if possible. For example, the design team may choose to have a central courtyard to minimize the floor plate depth and bring in additional light when forced to have a square-shaped plan. It should be noted that selecting a simple building shape would facilitate the construction, inspection and maintenance procedure and reduce the associated costs. 2.1.2

Window to wall ratio and daylight control strategies

Window systems have a major role in providing ample daylight and visual comfort to the occupants. For this reason, there has been a design trend in recent years towards highly glazed façades that provides the occupants with a sense of connection to the outside [40]. Despite the aesthetics, there are some disadvantages associated with fully transparent façades, including the relatively higher heat transfer of the façade and the complexities associated with daylight control. Moreover, these design solutions are not very environmentally friendly or cost efficient [39]. It is recommended to use a more solid, yet aesthetically pleasing solution rather than investing in expensive façade solutions as a means for mitigating heat gains and losses in highly-glazed façades. Moderate window to wall ratios (WWR) combined with high-performance window systems can allow for meeting occupant comfort requirements, as well as an optimal building energy consumption. Building codes and standard in each country provide recommendations regarding an optimal WWR. For example in Canada, the National Energy Code of Canada for Buildings (NEBC) [41] suggest the maximum allowable WWR be determined from Eq.1: = 0.4

=

2000 − 0.2 3000

= 0.2

where of the code.



< 4000 4000 <



< 7000

(1)

> 7000

is the heating degree days of the location of the building determined in accordance with the Sentence 1.1.4.1(1)

It is possible to offset the daylight load by the correct use of solar control solutions. Generally, solar control is achieved through the application of appropriate glazing units, shadings, and louvers. These provisions are normally implemented during the façade preliminary design phase. However, even in building design phase, appropriate application of window setbacks can deliver good shading potential as well as some attractive architectural features to the building.

Moreover, during the preliminary design phase of the building, it is recommended for the designer to allow for the building adaptation to various façade alternatives and detailing. This provides façade designers with the possibility of having more options and changing a selected alternative with another one, if needed, at minimum costs. 2.2 Façade preliminary design The façade preliminary design phase is the most important stage since the decisions made in this stage will directly influence the outcome and success of the later stages. Initially, the design criteria must be identified based on the project expectations and design attributes that were defined in the building design stage. It is necessary for the criteria to be exhaustive, meaning that consideration of these criteria will satisfy all intended performance attributes (if the requirements for other life cycle stages are met). The decision criteria to be considered for designing an optimal façade system are discussed in Section 3 with their assessment method. The next step involves the selection of feasible, desirable alternatives. The feasibility of the alternatives must be checked with related codes and standards that provide the minimum performance requirements of environmental separators. Also, strategies that can control solar radiation, heat transfer, air leakage and moisture migration must be considered in this step. 2.2.1

Solar radiation control strategies

Solar radiation can be controlled by various strategies. Some of these strategies such as building orientation and WWR need to be considered during the preliminary building design stage. However, the solar heat gain and the visible transmissibility of the window systems can be controlled in this phase to a higher extent. In this stage, according to the project needs, constraints (such as initial budget, maintenance needs), climatic conditions, and the expected performance level, the designer must decide on the type of windows (fixed or operable to provide natural ventilation), shadings (fixed or automated) or the glazing properties. For each climatic condition, the designer must decide on the number of glazing panes, the framing system, infilled gas(es) between glazing panes, coatings (type, color and glazing surfaces with the coating applied on them), based on the required thermal resistance, visual transmission, and heat gain properties. 2.2.2

Heat transfer control strategies

Heat transfer occurs through radiation, conduction, and convection. Hence, it is necessary to use a radiation barrier (solar control as explained above), thermal insulation, and air barrier systems. Application of thermal insulation is the most effective solution to control the heat transfer through the wall assembly. They increase energy efficiency by reducing the building’s heat loss or gain; control surface temperatures for occupant comfort and reduce the potential for condensation by controling temperatures within an assembly. Moreover, thermal insulations can sometimes add structural strength to a wall, such as in structural insulating panels (SIP), provide support for a surface finish, e.g. exterior insulation finish systems (EIFS), impede water vapour transmission and air infiltration, reduce noise and vibration, and reduce damage to structures from exposure to fire and freezing conditions. It must be noted that poorly designed or improperly installed thermal insulation may promote moisture condensation and subsequent damage within a building envelope. Thermal insulation materials are divided into four categories based on their physical structure and form, namely loose-fill, semi-rigid, rigid and formed-in-place insulation. The designer must select the most suitable insulation form, considering the envelope materials, construction requirements and their thermal resistance. 2.2.3

Air Leakage Control Strategies

Air leakage through the building envelope can cause several problems such as thermal discomfort, higher energy consumption, condensation, formation of ice dams on the roofs, durability issues, development of mold, noise transmission, odor, and poor indoor air quality. Air leakage occurs due to three driving forces namely, wind, stack effect, and combustion and ventilation. The paths for air leakage are: • • • • •

Cracks and joints between elements; Connection between wall and roof, wall and windows, etc.; Porous materials (e.g., concrete blocks, fiber boards ); Discontinuities in the air barrier; Openings for building services (pipes, electrical outlets, etc.).

To control air leakage, the designer must use a well-detailed, buildable and workmanship-tolerant air barrier system. It is important that the air barrier is continuous, structurally supported and durable. It is preferable to place the air barrier system

on the warm side of an insulated assembly, but this can be changed when it is suitable for a given construction practice, or due to the type of materials that are used. 2.2.4

Moisture control strategies

Moisture migration through the building envelope occurs due to rain penetration, air leakage and vapour diffusion. Rainwater penetration is the most important source of moisture problems in envelopes. The rainwater can penetrate the building when there is an opening in the envelope and a driving force to move the water through the opening. These driving forces include kinetic energy, surface tension, pressure assisted capillarity, gravity, and air pressure differentials. Strategies to control rainwater penetration include [42]: • • • •

Deflection using overhangs, balconies, or placing the wall to the orientation with least wind-driven rain exposure. Although the complete elimination of water on the envelope is not practical, the water sources can be greatly reduced. Elimination of openings by sealing all of the cracks or joints (i.e. face-sealed walls or perfect barrier walls). By controlling forces that drive rain penetration through rain screens, compartmentalization of the cavity, capillary break, etc. Proper drainage and application of storage wall systems.

To control moisture migration due to air leakage, an air barrier system must be used as explained earlier. In addition, another strategy to control the moisture migration through air leakage is the application of thermal insulation, because when the air leaks through the layers of the building envelope, condensation only occurs when the temperature of the layer is below the dew point. For this reason, proper application of thermal insulation can eliminate this problem. However, the control of air leakage is necessary for other reasons, as mentioned earlier in Section 2.2.3. Vapour diffusion in the envelope is the process by which water vapour migrates through the material, which is caused by the partial vapour pressure differential across the envelope. The moisture flux depends on the partial vapour pressure differential and the resistance of the material to moisture movement. To eliminate or more accurately, to retard the passage of moisture as it diffuses through the assembly of materials in a wall, a proper vapour barrier must be installed. It must be placed on or near the warm side of the insulation, which is normally the high vapour pressure side. The placement of vapour retarder should not prevent drying and the designer should avoid any “double-barrier” situation. 2.2.5

Selection of the most suitable alternative using decision support tools

In this step, the designer must select the most appropriate alternative from a pool of feasible design choices. For this purpose, it is necessary to compare these alternatives using a proper decision-making method. In a study by Moghtadernejad et al. [26], it was suggested that the Choquet integrals, is an appropriate decision-making method to be used in optimal façade design. The basis of Choquet is very similar to AHP, however, unlike all other multi-criteria decision-making (MCDM) methods, it has the capacity of accounting for interactions among criteria and the importance of each subset of criteria instead of a single one. To compare the design alternatives using a decision support tool, first the multi-criteria façade performance (MFP) matrix of the alternatives must be constructed and then an aggregation method is used to rank the alternatives. Eq. 2 and Eq. 3 demonstrate the aggregation functions for AHP and a 2-additive Choquet respectively.

=

(2)

where are elements of the normalized MFP matrix for each alternative, and is the weight assigned to the ! criterion which ranges between [0, 1], using pairwise comparisons and calculating the priority vector (See [26, 43] for more detailed explanation).

"

=

# $ % −

1 2

* ,(,⊆.

' ()

−

()

(3)

where $ is the importance index and '#μ, 0% is the interaction index between criteria and 0 which ranges between [-1, 1]. For two criteria 1 and 1( , when the interaction index '#2, 0% = 0, the criteria are independent. Hence, from Eq.3, it is obvious that when the criteria are independent, the assessment of the alternative is by a simple weighed sum (similar to AHP). '#2, 0% > 0 when there is a complementary interaction among 1 and 1( , meaning that both criteria must be met to get a satisfactory alternative. If '#2, 0% < 0 then there is a substitutability or redundancy among 1 and 1( . This implies that

the satisfaction of one of the two criteria is sufficient to have a satisfactory alternative. In a previous study, the interactions among 15 façade design criteria were identified using a principal component analysis approach [43]. The decision-making process is further explained in Section 3 of this paper, where four façade design alternatives, are assessed and compared with regards to the 15 decision criteria; and consequently, the most suitable alternative is selected. 2.3 Façade detailed design 2.3.1

Design for various loads

In the detailed design stage, the designer or the structural engineer must design the building envelopes for [44]: • • • • • •

Dead loads (self-weight) of the assemblies Hydrostatic and earth pressure loads Air pressure loads on the air barrier Loads due to thermal or moisture-related expansion and contraction, deflection, deformation, creep, shrinkage, settlement, and differential movement Wind loads (including the up-lift imposed on roofing), snow, rain Seismic loads

When the system is designed for dead loads, earth pressure loads, hydrostatic loads, thermal and moisture expansion and contraction and air pressure, the components of the envelope must remain in the elastic range and comply with lowerbound material properties for design [45]. The façade connections are designed in proportion to their tributary load. To design for wind loads, the equivalent static peak wind force is considered, and the components are designed to remain in the elastic range. The design should also meet strength and serviceability requirements of the code. In some regions, it might be necessary to design for missile impacts related to hurricanes and tornados [45]. The current codes and standards estimate these loads on the basis of past experience. However, these estimates do not account for wind loads in every situation and have some shortcomings [37, 46]. For example, evaluation of wind loads for many building shapes are not covered by standard shapes. Other shortcomings of codes relate to the effect of neighboring buildings on wind loads and pressure equalization, which may reduce but mostly increase wind loads. However, it is possible to determine the performance of façade elements by appropriate wind tunnel tests. Seismic loads are dynamic in nature, but equivalent static loads are normally considered in design and the components are to remain in an elastic state for these loads. However, seismic design also requires additional detailing requirements and factors to ensure that connections perform adequately and can accommodate the applied loads deformations. In some buildings, along with designing the building envelope for conventional loads, it is necessary to consider the blast impact as well. While designing for blast loading, the designer must consider two factors: the static increase factor (SIF) used for factoring the lower bound strength in determining the required material strengths and the dynamic increase factor (DIF) to include loading rate effects on the various material characteristics [45]. In designing for blast loads, the components can undergo inelastic deformations since the intention of such provisions is to prevent any loss of life during the impact. Hence, the connections must be designed for the out of plane ultimate flexural capacity of the attached components. In the traditional design approach, the designer considers each load case independently. In such approaches, the design can undergo some necessary iterations (since the synergy or conflicts among various design cases are not considered). In an efficient design, it is necessary to consider all detailed design criteria and their interactions during each step, i.e., dividing the detailed design phase by various tasks rather than by discipline. McKay et al. [45] provide two sets of flowcharts that demonstrate the traditional (ineffective) and the recommended (effective) design procedure as demonstrated in Figure 2 and Figure 3.

Figure 2. Traditional design process, adapted from [45]

Figure 3. Recommended design process, adapted from [45]

2.3.2

Necessary considerations during detailed design

It is necessary to allow for construction alterations, such as when alignment during erection changes from the original design. Moreover, the connections should be able to accommodate the tolerances associated with the erection process. Other considerations in this phase include some durability considerations in detailing of the façade and its connections, such as details related to caulking, expansion joints, the appropriate number of joints (to eliminate water infiltration). It is the responsibility of the designer to ensure the constructability (ease and efficiency of the construction phase) and inspectability of the design during the pre-construction phase. This includes identification of the obstacles to eliminate or reduce errors and delays or unexpected costs. It is also recommended to provide a maintenance, inspection and end-of-life plan for the building envelope at this stage. In such plans, the inspection intervals are determined by considering the worst-case scenarios in the combinations of façade degrading factors. Application of a building information model (BIM) can significantly facilitate the integration of the tasks from the design stage, with construction and maintenance phases. BIM provides a reliable basis for building life cycle decision-making from the conception phase to the final demolition, in the form of a shared information resource thereby facilitating the cost evaluation, construction and project management processes [37, 47, 48]. Using these models help data losses that usually occur when a new team takes over the project and delivers more detailed and comprehensive information on complex projects. 2.4 Construction and maintenance considerations Although the design of the façade system is completed at the end of the façade detailed design stage, to ensure an optimal and high-performance façade system, some actions and considerations are necessary throughout the rest of the façade life cycle that are briefly discussed as follows. 2.4.1 Façade construction It is important to ensure the constructability of a façade system (i.e. construction flexibility, proper detailing and alignments, availability of materials and components, attainable workmanship, proper sequencing, and seasonality), as it is an important factor in optimal design procedure. Correct installation of façade panels is a key element in ensuring façade integrity and good performance [38, 49, 50]. Presently, with advances in technology and the importance of fast construction with appropriate quality control, designers favor prefabricated systems. The drawbacks of prefabricated construction include the small error margin, lack of design flexibility, complexity of connections and necessity of bracing for installation. Moreover, before commencing the prefabrication procedure, the manufacturer must check the compatibility of the façade system with the as-built details and dimensions of the building frame. The provisions of an optimal design can be attained through good communication and a clear definition of the responsibilities of the design and construction teams, competent workmanship and quality control. Although the last two can only be conceived through accurate detailing and clear specifications of the involved systems. This emphasizes the necessity of selecting simple and executable systems and the related connections in the design phase. Appropriate testing facilities must be present to test and approve the façade assemblies before being installed on-site. It is preferable to hire the façade designer for the inspection and the quality control procedure both during and after the completion of construction work [51]. 2.4.2 Façade operation and maintenance The degradation mechanisms commence within a short time after completion of the construction and will gradually influence the façade performance. An optimal façade system will maintain its aesthetical and expected performance, with minimal maintenance costs. It is necessary to conduct regular cleaning and inspections to ensure the proper performance of façades throughout their life cycle [52]. The simplicity of the selected system and ease of access to various components and elements of the façade system can significantly reduce the required time for the maintenance work and consequently decrease the associated costs. Hence, it is important to consider these factors in the design stage. 2.4.2.1 Façade cleaning The cleaning requirements of façades normally depend on aesthetic demands, the function, and location of the building, and atmospheric conditions [53]. In addition to aesthetics, which is the principal reason for cleaning building façades, it is possible to prevent the acceleration of façade deterioration by removing pollutants, such as sulfur and acid rain impurities. Moreover, façade cleaning allows for better condition assessment of the system; hence, it is more sensible to perform façade cleaning before any inspections or repair work.

The cleaning work must be performed after some necessary preliminary tasks such as protecting building materials that could be damaged by cleaning and test-cleaning on a small patch of the façade. Cleaning methods include chemical, non-chemical or water cleaning, abrasive and a hybrid approach that applies a combination of these techniques [37]. Each façade material needs an appropriate cleaning method that is determined by the cleaning agency in consultation with the design professionals. 2.4.2.2 Façade inspection Several cities in north America (e.g. New York, Chicago, Montreal) have by-laws requiring regular inspections of the façade systems due to the past incidents that caused injuries or deaths [50, 54, 55]. Regular inspections should be performed on the basis of the maintenance requirements for each façade assembly. The façade condition assessments are typically performed in three steps [37]. First, the related façade documents are reviewed through data from BIM or other available documents, and as-built drawings are prepared where these documents are not provided. Afterwards, a visual inspection is performed under appropriate lighting conditions, to ensure all areas of the façade system are clearly visible and not obscured by sunlight or shading at certain angles [56]. Inspectors are able to detect the evident deteriorations such as cracks and spalls and movements of the various elements by visually inspecting the façade system; however, they cannot detect the deteriorations that are developing or hidden signs of defects such as the connections that are normally hidden from view. Consequently, it is required to perform a regular close-up and detailed inspection of the façade system. Such assessments are performed by probing of the elements to detect hidden deteriorations or using scaffolding and other appropriate tools such as thermal imaging or laser assessment that are utilized to examine the hidden parts of façades or elements with limited accessibility. Consequently, the inspector evaluates the condition of the façade system and reports the results to the building owner and the local building authority and based on the results, the required maintenance work is arranged. It is important to keep these records throughout the building service life for any future assessments. 2.4.2.3 Repair and rehabilitation and strengthening The service life expectancy of buildings are approximately 60 years; however, this is generally lower for the building envelopes, including the façades. As a result, to restore the safety and serviceability of the façades to their approximate original condition, some corrective maintenance work must be carried out. The corrective maintenance can be in form of repair, rehabilitation or strengthening [37, 40]. It is necessary to execute the required maintenance work immediately after the diagnosis of defects and deteriorations since deferring them can increase the related costs due to accelerated rates of deterioration [57]. In severe cases, deferred maintenance can lead to large economic losses, disastrous failures involving injuries and even death. 3

Case study: Selection of the most appropriate alternative using decision support tools

In this section, four façade alternatives (combination of two wall systems and two window systems as presented in Table 2) are selected for a two-story commercial building to be built in Montreal’s downtown. These alternatives are assessed with regards to 15 decision criteria and the MFP matrix is constructed. Subsequently, the alternatives are ranked using Choquet integral and AHP and the results are compared. These two methods are similar, except that Choquet integral can account for the interactions among criteria and hence avoid the double-counting issue that is present in current decision-making tools. In this procedure, the decision-maker tries to select the design criteria to be exhaustive (i.e satisfying all required performance attributes) while being as independent as possible to minimize the double counting issue of the AHP method. The area of the building per floor is 930 m2 and the overall window to wall ratio for all design concepts were selected to be 40%, which is the maximum recommended ratio defined by the NEBC [41]. The window systems, thickness and position of the insulations, vapour barriers and air barriers were selected in accordance with the needs of Montreal weathering conditions which fall under cold climate (zone 6) category according to ASHRAE Handbook of Fundamentals [58]. The wall systems (brick and fibercement) were checked with WUFI [59] to check their compatibility with Montreal weathering conditions for a two-year-period starting 1/10/2017 to 1/10/2019. The results of the simulations indicate no potential moisture or condensation problem in the walls. The total water content is decreasing for both brick and fibercement walls and the moisture content and the relative humidity profile of the moisture-sensitive layer, i.e. plywood, for both brick and fibercement alternatives are acceptable since the moisture content is below 20%, the relative humidity does not reach 100% and the dewpoint is always below the layer temperature (Figure 4 and 5). 3.1 Data collection To compare these four alternatives, the performance of each alternative is assessed with respect to each decision criterion. Table 3 presents the 15 decision criteria and the assessment methods for evaluating the performance of each alternative. For

calculation of the overall thermal resistance of the alternatives, the thermal resistance of the walls and window systems have been calculated separately and the overall resistance was determined by calculating the weighted average (60% wall, 40% windows) of the results. Figure 6(a) and 6(b) demonstrate the thermal analysis of the walls using THERM [60] simulations. According to the results of the simulations, the thermal resistance of the brick and fibercement wall systems are 3.65 and 3.16 #

34 5 6

% respectively. Similarly, THERM-Window simulations were performed on the triple and double pane window systems 34 5

and the overall thermal resistance of the triple and double pane window systems are determined to be 1.05 and 0.6 # % 6 respectively (Figure 7). The visible transmittance (VT), solar heat gain coefficient (SHGC) and the condensation resistance (CR) of the window systems are also derived through the THERM-Window simulations. The overall window performance is the average of the normalized VT, SHGC and CR values. Table 4 summarizes the results of performance assessment of each alternative with respect to 15 design criteria. Table 2. Specifications of four façade alternatives for a low-rise commercial building Alternative 1 Wall Metal framing 25ga. 6" NLB, 24 OC • 100mm exterior brick • 40mm air space • One layer of Tyvek weather-barrier membrane • 12.5mm plywood, exterior grade • 150mmglass fiber insulation • 3mm PE membrane, VB • 12.5mm inch gypsum board Alternative 3 Wall Metal framing 25ga. 6" NLB, 24 OC • Fibercement • 25mm air space • One layer of Tyvek weather-barrier membrane • 12.5mm plywood, exterior grade • 150mmglass fiber insulation • 3mm PE membrane, VB • 12.5mm inch gypsum board

Window 2 layers of 4mm Low-E glass (surface 2 and 5), clear 4mm glass in between, 12mm argon space. Timber and aluminum frame.

Window 2 layers of 4mm Low-E glass (surface 2 and 5), clear 4mm glass in between, 12mm argon space. Timber and aluminum frame.

Alternative 2 Wall Metal framing 20ga. 6" NLB, 24 OC • 100mm exterior brick • 40mm air space • One layer of Tyvek weather-barrier membrane • 12.5mm plywood, exterior grade • 150mmglass fiber insulation • 3mm PE membrane, VB • 12.5mm inch gypsum board Alternative 4 Wall Metal framing 25ga. 6" NLB, 24 OC • Fibercement • 25mm air space • One layer of Tyvek weather-barrier membrane • 12.5mm plywood, exterior grade • 150mmglass fiber insulation • 3mm PE membrane, VB • 12.5mm inch gypsum board

Window

2 layers of 4mm LowE glass on surface 3, 16mm argon space. Timber and aluminum frame.

Window

2 layers of 4mm LowE glass on surface 3, 16mm argon space. Timber and aluminum frame.

Table 3 Criteria to consider in the preliminary façade design stage and summary of their assessment method Design Criteria Thickness

SI Units M

Weight

kN/m2

Fire Rating

Minutes

Vapor resistance

ng/Pa·s·m2

Thermal resistance

RSI (m2K/W)

Noise reduction

STC

Window performance • Visible transmission (VT) • Solar heat gain coefficient (SHGC) • Condensation resistance (CR) Ease of construction

Points Labour hours/m2

Energy consumption (cooling/heating/lighting)

kWh/m2

System effect on the environment • Global warming potential • Acidification potential • Human health (HH) criteria • Eutrophication potential • Ozone depletion potential • Smog potential • Total primary energy a. Non-renewable energy b. Fossil fuel consumption Expected service life

Points kg CO2 eq /m2 kg SO2 eq/m2 kg PM2.5 eq/m2 kg N eq/m2 kg CFC-11 eq/m2 kg O3 eq/m2 MJ/m2 MJ/m2 MJ/m2 Years

Initial cost (design and construction) Operation and maintenance cost Decommissioning cost

$/ m2 $/ m2 $/ m2

Aesthetics

Points

Reason for consideration and assessment method Thinner walls are desired to maximizing living space. The thicknesses of the wall assemblies are considered. Lighter systems allow for ease of construction, maintenance and decommission. The weighted average (60% walls and 40% windows) weight of the wall and window assemblies are considered. Fire rating of the wall assemblies are considered in accordance with Table 9.10.3.1-A of National Building Code of Canada (NBC) [61], Chapter 7 of the International Building Code [62], the Fire Ratings of Archaic Materials and Assemblies [63]. It must be noted that for this specified building the minimum required fire rating of the exterior walls is 45 minutes. Higher resistance is required to control indoor air quality and avoid moisture damage. The vapour resistance of each material is the inversion of its permeability. The overall vapour resistance is derived from the summation of the vapour resistance of each component (see Chapter 26 of ASHRAE handbook of fundamentals [58]. Higher resistance is required to prevent heat transfer mechanisms. Overall thermal resistance is determined using THERM-Window [60] simulations. The enclosure must be effective in attenuating airborne sound. The weighted average of the Sound Transmission Class (STC) of the system is considered. The STC of the window systems are provided in the manufacturer’s manual and the walls system STC is determined in accordance with ASTM E413 – 16 [64] and NBC [61]. Window performance indicates the amount of visible radiation passing through the fenestration system, the solar heat gain, and condensation resistance of the window system. These attributes (VT, SHGC, and CR) are calculated using the THERM-window simulations. The goal is faster construction (also results in lower construction costs). The labour hours per square meter construction of each wall and window assembly is considered and then the weighted average is calculated. The goal is to design energy efficient building enclosures to avoid energy waste. The annual energy consumption is calculated using eQUEST [65] simulation tool. The goal is to have a minimum adverse effect on the surrounding environment. Life cycle Assessment of each system is performed using Athena Impact Estimator [66]. The assessed factors are calculated from the resource extraction phase to building demolition and materials end-of-life disposition (disposal or transfer for recycling or reuse).

The expected service life of a material in certain weather conditions considering no undue damages will occur. The service life estimation was conducted in consultation with CSA S478-1995 guideline on durability in buildings [67], and the Delphi study report published by Canada Mortgage and Home Corporation [68]. Life cycle costs should be considered to havean idea of the investment return time. Initial and demolition costs are determined by consulting RSMeans 2017 Building Construction Cost Data [69], and RSMeans 2017 Assemblies Costs Book [70]. The maintenance costs were estimated using the information from the expected service life of the components and their maintenance needs [67, 68] and using the related repair/replacement costs in consultation with data provided in RSMeans 2017 Building Construction Cost Data [69]. Depends on the stakeholders’ or designer’s preferences and subjective opinion.

Plywood, Exterior-Grade

Water Content [kg/m3]

Water Content [M.-%]

Water Content [kg/m3]

Total Water content

(b)

Plywood, Exterior-Grade

Plywood, Exterior-Grade

Temperature [

Temperature [

]

Relative Humidity [%]

]

(a)

(c)

(d)

Figure 4. The overall moisture content of the brick wall and the moisture and relative humidity profile of the moisture sensitive layer

Water Content [kg/m3]

Plywood, Exterior-Grade

Water Content [M.-%]

Water Content [kg/m3]

Total Water content

(b)

Plywood, Exterior-Grade

Plywood, Exterior-Grade

Temperature [

Temperature [

]

Relative Humidity [%]

]

(a)

(c)

(d)

Figure 5. The overall moisture content of the fibercement wall and the moisture and relative humidity profile of the moisture sensitive layer

Inside temperature 22

Outside temperature -5 (a) Inside temperature 22

Outside temperature -5 (b) Figure 6. Thermal analysis of the brick (a) and fibercement (b) wall assemblies

(a)

(b) Figure 7. Thermal analysis of the triple pane (a) and double pane (b) window systems

Table 4 Performance assessment of the alternatives Design criteria

Pref.

Assessed performance of the alternatives

A2 A1 Total thickness L 0.322 0.322 Weight L 1.41 1.37 Fire rating H 120 120 Vapor resistance H 6.484 6.484 Thermal resistance H 2.61 2.43 Sound transmission class H 49.6 46.4 Window solar performance H 19.894 21.316 H 0.432 0.569 • VT H 0.25 0.378 • SHGC H 59 63 • CR 8 Ease of construction L 1.97 1.94 9 Annual energy consumption L 132.43 133.37 10 System effect on the environment L 20.399 16.688 L 3.25 2.463 • GWP L 0.039 0.031 • Acidification L 0.004 0.003 • HH criteria L 0.002 0.002 • Eutrophication potential L 1.86E-07 1.29E-07 • Ozone depletion potential L 0.497 0.425 • Smog potential L 75.9 62.146 • Total primary energy a. Non-renewable energy L 53.8 44.427 b. Fossil fuel consumption L 50.1 40.695 11 Durability H 50 50 12 Initial costs L 509.67 440.14 13 O&M costs L 479.27 409.74 14 Decommissioning costs L 14.21 14.08 15 Aesthetics H 0.35 0.35 “H” denotes a higher value is preferred while “L” means a lower value is desirable 1 2 3 4 5 6 7

A3 0.219 0.42 120 6.468 2.316 46.6 19.894 0.432 0.25 59 1.02 132.76 14.746 1.765 0.025 0.005 0.002 1.88E-07 0.362 58.341 37.972 34.238 30 408.6 611.35 9.25 0.65

A4 0.219 0.38 120 6.468 2.136 43.4 21.316 0.569 0.378 63 0.99 133.66 11.033 0.981 0.017 0.004 0.002 1.31E-07 0.29 44.571 28.559 24.872 30 339.06 541.81 9.2 0.65

A1 III IV I I 1 1 III

Ranked performance of the alternatives A2 A3 III I III II I I I III II III III II I III

A4 I I I III IV IV I

I I IV

II III III

III II II

IV IV I

I IV II IV III

I III I III III

III II IV II I

III I III I I

3.2 Data processing It can be noted from Table 4 that each design criterion has a different unit and order of magnitude. To avoid the excessive influence of criteria with larger values, the data is normalized. Eq. 4 and 5 are used to normalize the values when a higher value is desired (such as thermal resistance) and when a lower value is desired (such as costs), respectively. 7( 7∗( = (4) 97:#7 % 7 ∗( =

9 ; #7 % 7(

(5)

where 7 ( is the assessed value for the ! criterion of each alternative 0. In addition, as a requirement for AHP summation of the assessed alternative values for each criterion should be equal to 1. Hence all 7∗( s must be multiplied by a normalizing factor as presented in Eq. 6. Table 5 demonstrates the normalized MFP matrices for AHP and Choquet. ∗ ( #

%

=

∑3 (

1

7∗(



(6)

Table 5 Normalized MFP matrices for Choquet and AHP AHP # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Criteria Total thickness Weight Fire rating Vapor resistance Thermal resistance Sound transmission class Window solar performance Ease of construction Annual energy consumption System effect on environment Durability Initial costs O&M costs Decommissioning costs Aesthetics

A1 0.202 0.110 0.250 0.250 0.275 0.267 0.220 0.169 0.251 0.198 0.312 0.204 0.260 0.196 0.175

A2 0.202 0.113 0.250 0.250 0.256 0.249 0.280 0.171 0.249 0.245 0.312 0.236 0.305 0.198 0.175

A3 0.298 0.369 0.250 0.250 0.244 0.250 0.220 0.325 0.250 0.240 0.188 0.254 0.204 0.302 0.325

A4 0.298 0.408 0.250 0.250 0.225 0.233 0.280 0.335 0.249 0.316 0.188 0.306 0.231 0.304 0.325

Choquet A2 A3 0.680 1.000 0.277 0.905 1.000 1.000 1.000 0.998 0.931 0.887 0.935 0.940 1.000 0.786 0.510 0.971 0.993 0.998 0.733 0.730 1.000 0.600 0.770 0.830 1.000 0.670 0.653 0.995 0.538 1.000

A1 0.680 0.270 1.000 1.000 1.000 1.000 0.786 0.503 1.000 0.598 1.000 0.665 0.855 0.647 0.538

A4 1.000 1.000 1.000 0.998 0.818 0.875 1.000 1.000 0.991 0.971 0.600 1.000 0.756 1.000 1.000

Preference weights or importance weights, are determined by using the pairwise comparison of decision criteria. In this process the decision-maker would assign a value from 1 as “equally important” up to 9 for “extremely more important” to 1 ( while comparing the relative preference of criteria to 0. This comparison is based on the project priorities and preferences with respect to the importance of each criterion. Table 6 illustrates the pairwise comparisons of the 15 design criteria, by the decision-maker. The weight of each criterion is obtained by normalizing the elements of the first Eigenvector as shown in Table 7. Table 6 Pairwise comparison of decision criteria c1 c2 c3 c4 c5 c6 c7 c8 c9 c10 c11 c12 c13 c14 c15

c1

c2

c3

c4

c5

c6

c7

c8

c9

c10

c11

c12

c13

c14

c15

1.000 3.000 0.333 5.000 5.000 3.000 5.000 3.000 9.000 9.000 7.000 5.000 5.000 5.000 9.000

0.333 1.000 0.333 5.000 5.000 3.000 3.000 3.000 9.000 7.000 5.000 5.000 5.000 5.000 7.000

3.000 3.000 1.000 5.000 5.000 3.000 3.000 5.000 9.000 9.000 7.000 5.000 5.000 5.000 5.000

0.200 0.200 0.200 1.000 1.000 0.200 0.200 0.200 7.000 5.000 3.000 1.000 1.000 1.000 1.000

0.200 0.200 0.200 1.000 1.000 0.200 0.143 0.200 9.000 7.000 5.000 1.000 1.000 1.000 0.333

0.333 0.333 0.333 5.000 5.000 1.000 1.000 5.000 9.000 9.000 9.000 5.000 5.000 5.000 5.000

0.200 0.333 0.333 5.000 7.000 1.000 1.000 1.000 7.000 9.000 7.000 3.000 3.000 3.000 3.000

0.333 0.333 0.200 5.000 5.000 0.200 1.000 1.000 7.000 9.000 7.000 3.000 3.000 3.000 5.000

0.111 0.111 0.111 0.143 0.111 0.111 0.143 0.143 1.000 0.333 0.333 0.200 0.333 0.200 0.333

0.111 0.143 0.111 0.200 0.143 0.111 0.111 0.111 3.000 1.000 0.200 0.333 0.333 0.333 0.333

0.143 0.200 0.143 0.333 0.200 0.111 0.143 0.143 3.000 5.000 1.000 0.333 0.333 0.333 0.333

0.200 0.200 0.200 1.000 1.000 0.200 0.333 0.333 5.000 3.000 3.000 1.000 1.000 1.000 3.000

0.200 0.200 0.200 1.000 1.000 0.200 0.333 0.333 3.000 3.000 3.000 1.000 1.000 1.000 3.000

0.200 0.200 0.200 1.000 1.000 0.200 0.333 0.333 5.000 3.000 3.000 1.000 1.000 1.000 3.000

0.111 0.143 0.200 1.000 3.000 0.200 0.333 0.200 3.000 3.000 3.000 0.333 0.333 0.333 1.000

Table 7 Preference weights of design criteria c1

c2

c3

c4

c5

c6

c7

c8

c9

c10

c11

c12

c13

c14

c15

0.011

0.014

0.010

0.057

0.067

0.017

0.022

0.025

0.229

0.190

0.126

0.051

0.052

0.051

0.078

The priory vector in Table 7 indicates that energy efficiency (1= ), environmental impacts#1 > ), life cycle costs (summation of 1 ? , 1 @ , 1 A ), and durability #1 ) have the highest weights according to the decision-maker. To ensure that the decisionmaker has been consistent with assigning preferences, the consistency ratio B should be less than 0.1 [43, 71]. The consistency ratio, is calculated, as indicated in Eq.7 and 8. B' =

C3DE − ; 1.93 = = 0.137 ;−1 14

(7)

B' 0.137 = = 0.087 ' 1.583

(8)

B =

where B' is the consistency index, C3DE is the largest Eigenvalue, and ' is the random consistency index, which is equal to 1.583 , for ; = 15 [72].

The interaction indices among the criteria were identified in a previous study [43] using principal component analysis (PCA) in an unsupervised approach; and the results are summarized in Table 8. It is observed that most of the interactions are not high which confirms that the decision criteria have been selected to be almost independent. The highest interaction is 'I,= = −0.064, between the thermal resistance (1I % and annual energy consumption #1= %, which is rational as the thermal resistance of a façade assembly can affect the energy performance of the building. Other relatively high interactions are 'K,L = −0.057, 'A, ? = −0.043 and '@,= = −0.045. Table 8 Interaction indices among design criteria, adopted from [43] Crite ria c1

c1

c2

-

-0.019

-0.030

-0.006

-

-0.014

-0.007

-

-0.011

-0.019

-0.012

0.004

-

0.004

-0.035

0.004

-

-0.008

-0.033

-

c2 c3

c3

c4

c4

c5 c6 c7

c5

c6

c7

c8

c9

0.003

0.003

0.002

-0.004

-0.012

-0.031

-0.017

-0.013

-0.010

-0.028

0.001

-0.001

-0.006

0.000

-0.031

-0.005

-0.017

-0.026

-0.014

-0.029

-0.028

-0.001

0.000

-0.045

-0.037

0.000

0.001

-0.001

-0.012

-0.003

0.001

-0.009

0.002

-0.027

-0.043

-0.023

-0.007

-0.015

-0.006

-0.064

0.004

-0.003

-0.008

-0.004

-0.002

0.001

-0.057

-0.018

0.006

-0.027

-0.004

-0.001

-0.009

-0.008

-0.004

-

0.002

-0.037

-0.034

0.001

-0.011

-0.018

0.002

0.001

-

-0.004

-0.006

-0.019

-0.006

-0.022

-0.035

0.000

-

-0.003

-0.002

-0.018

-0.003

-0.008

-0.002

-

-0.004

-0.008

0.000

-0.020

0.000

-

-0.034

-0.034

-0.018

-0.005

-

-0.017

-0.012

-0.010

-

-0.019

-0.005

-

0.000 -

c8 c9 c10 c11 c12

c10

c11

c12

c13 c14 c15

c13

c14

c15

Finally, the alternatives are ranked using the aggregation functions demonstrated in Eq. 2 and 3. While Choquet method used both importance weights and interaction indices, the AHP method only considers the importance weights. The results are summarized in Table 9. Table 9 Concept scores and rankings of the four alternatives using AHP and Choquet methods Alternatives A1 A2 A3 A4

AHP score 0.2345 0.2465 0.2498 0.2692

Rank IV III II I

Choquet score 0.9639 0.9871 0.9519 0.9907

Rank III II IV I

Rank change in AHP & Choquet 1 1 -2 0

3.3 Comparison of the results Although the design criteria were selected to be almost independent, some rank alternations are observed. While both methods rank A4 as the most suitable alternative, the ranking changes for the rest of alternatives depending on the method of comparison. This means that in other design cases, the outcome of the alternative selection (rank I) could be different using AHP and Choquet. It is noted that both AHP and Choquet share a weighted mean operator; however, the ability of the Choquet method in considering the interactions among criteria, results in the rank alternations where A2 is selected as the second-best option while AHP selects A3 as the second-best option. This confirms that even very small dependencies among design criteria can result in double-counting issues in ranking the alternatives. Choquet integral not only considers the importance of each criterion (as in AHP) but also considers the importance of each subset of criteria [73]. In practice, façade design criteria are not independent, and when two criteria are interacting, such as thermal resistance and annual energy consumption, when only the weighted partial scores of the criteria are combined, their scores will be double counted. Moreover, in the decision-making process, presence of some criteria may not contribute individually to the total score by themselves, but in conjunction with other criteria, the total score may rise sharply. For example, in façade design, a very high score in vapour resistance of system may not be important by itself if the operations and maintenance (O&M) costs of the system are not influenced by it, but in combination, it may significantly contribute to the total score of an alternative. This emphasizes the benefit of using the Choquet method over other methods that require the design criteria to be independent.

Summary and conclusions Over the past decade, a new trend has emerged toward the development of sustainable building design and construction, which has caused an increased focus on designing high-performance building structures. Amongst all these building structures and components, façades have the potential to drastically affect the comfort level of occupants, energy performance and the environmental footprint of buildings; therefore, more attention and effort needs to be dedicated to their design and construction. Like most modern technological developments, various disciplines and knowledge bases are involved in the process of designing façade systems. The multidisciplinary nature of façade design, in addition to the urge for satisfying distinctive design and performance criteria, cause the design process to become considerably complex. This complexity is more tangible during the integration, where a balance should be maintained between all necessary performance criteria, which can be conflicting with each other. Hence, there is a need for a systematic approach to account for provisions in all design stages and provide the required balance. MCDM methods are useful tools in assisting designers in façade preliminary design stage. Although the literature suggests AHP is the most suitable and commonly used method, the authors believe that this method does not reflect the most precise evaluation of alternatives’ performance since it cannot account for the interactions among design criteria. This is problematic since even though designers select the criteria to be independent, in real life design some interactions will always remain and can result in rank alternations where the criteria assessments for alternatives are close to each other. Among all decision analysis functions and aggregation operators, Choquet is the only decision-making method capable of considering such interactions. In this paper, a relatively new and useful, systematic approach is proposed to support the design of the optimal façade systems using multi-criteria decision support tools. To this end, major metrics and criteria which constitute a highperformance façade system were defined and the designers were provided with an interactive and comprehensive guideline for façade design which outlines the necessary considerations for each stage of the façade life cycle. Subsequently, in a case study, four façade alternatives were ranked using AHP and Choquet integrals and the results were compared. The results confirm that even very small interactions among criteria can result in rank alternations, and to have an unbiased comparison of the alternatives it is necessary to use a decision-making method that can account for such interactions. Acknowledgments The authors would like to acknowledge the financial support of the Fonds de Recherche du Québec - Nature et Technologies (FRQNT), the Natural Sciences and Engineering Research Council (NSERC), Sheryl and David Kerr Engineering Graduate Studies Fund, and the Department of Civil Engineering at McGill University. References [1] [2]

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Highlights: • • • • •

Introducing a systematic and integrated approach for designing high performance and optimal building façades. Identifying necessary facade considerations during various life cycle stages. Identifying major performance attributes of building facades and the related quantification measures. Application of Choquet integral in decision making related to system selection and ranking of the alternatives which is unprecedented in building science applications. Avoid time-consuming and costly iterations related to façade design.

AUTHOR DECLARATION We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. We understand that the Corresponding Author is the sole contact for the Editorial process (including Editorial Manager and direct communications with the office). She is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. We confirm that we have provided a current, correct email address which is accessible by the Corresponding Author and which has been configured to accept email from: [email protected]

Authors: Saviz Moghtadernejad, PhD McGill University [email protected] Prof. Luc Chouinard, Associate Professor McGill University [email protected] Prof. Saeed Mirza, Emeritus Professor McGill University [email protected]