A performance based decision-making approach for insulation material selection: A social housing case

Built environments have been evolving since the 500s BC, as we can observe aesthetic, social and environmental concepts in ancient Greek cities' performances. Performance can be explained as the ability to perform something, or the fulfillment of an obligation or a claim. The three principles of architecture; firmitas (durability), utilitas (utility) and venustas (beauty), introduced by Vitruvius (1960), are also early examples of the built environment and the evolution of its performance. Built environments and their performance have a great impact on human life, as we spend most of our times in them. Moreover, the performance of buildings have been frequently discussed in the last decades, mostly in terms of sustainability, energy, cost, health, etc. Buildings, as the consumers of more than one third of global energy, have a significant impact on global warming, the depletion of resources, and emissions. The degree of environmental impact caused by buildings and their energy consumptions has been increasing rapidly, due to rising comfort needs and population growth. The high amount of energy consumption in buildings has led to the need for energy efficiency action in terms of buildings through legislative strategies such as energy efficiency regulations, energy certification of buildings according to energy classes, refurbishment of existing buildings, etc. On the other hand, building design involves several decisions in different stages of the build process. Building design, in the form of the process of decision-making, involves decision makers such as investors, property owners, designers, experts, etc., with each decision maker, from different points of view, targeting the success of the project in terms of different concerns involving several dependent and independent parameters. With the increase in building energy consumption levels worldwide, thermal insulation materials have become one of the key applications for building energy efficiency. When high performance buildings are targeted, thermal insulation materials become increasingly significant. Consequently, conflicts may occur between the efforts and the provisions due to thicker applications of the material, leading to higher costs and greater environmental impact. In addition, higher comfort levels are also expected as a feedback for such intensive efforts. Thus, the determination of the most effective thermal insulation material in terms of its properties for high performance building is a multiple criteria decision-making problem, since the thermal insulation materials are significant in terms of their impact on the energy, cost, thermal comfort, and environmental performance of a building. Urbanization and social housing has an important impact on the embodiment of 'housings'. The requirements of social housing such as low investment cost for states and low operating costs for low-income group occupancies signify the importance of performance estimation for social housing. From this point of view, one of the most significant performance indicators for social housing are cost and energy, since they are also significant with regard to current directives and regulations. Besides, a comprehensive definition for social housing performance within a sustainability approach has an obvious importance for economic, social and environmental development. Nevertheless, current regulations and developments also regularly identify new criteria in order to expand the definition of building performance. EN 15459 (2017) introduced new revisions such as the cost of greenhouse gas emissions in order to evaluate the monetary value of environmental damage caused by CO2 emissions related to the energy consumption of buildings. Moreover, environmental product declarations (EPD) came into prominence in order to verify the information about the life-cycle environmental impact of products. EPBD (2018) also states the importance of building energy performance improvements while improving thermal and visual comfort. Thus, in addition to energy and cost performances, the environmental impact and user comfort performances should be also considered as determinative and descriptive for a sustainable social housing design. Environmental impact has been discussed and evaluated since the 1960s. By the publication of the National Environmental Policy Act (NEPA, 1970) in U.S., environmental impact assessment obtained formal status. Today, environmental impact is evaluated either on material, component or building scale, depending on the purpose of the evaluation. Thermal comfort is also a Twentieth Century discipline, focusing on a comfortable indoor environment in terms of thermal criteria. As a result of the evolution of space phenomenon and architectural theories and movements, occupancy became an important factor as an individual with a physical dimension in the design process. This evolution brought the need to evaluate and increase user comfort within the built environment. In this study, "high performance" is evaluated in terms of energy, cost, thermal comfort, and environmental impact. Thus, the aim and scope of the study, beyond a consideration of cost optimal energy efficiency, is proposing a methodological approach for a performance-based decision-making method covering energy, cost, thermal comfort, and environmental performance. Within this context, the methodological approach presented in this study includes the following main steps: • Determination of the archetype • Parameterization with the designated independent variables • Calculation of the dependent variables • Implementation of a multiple criteria decision-making process • Determination of the best alternatives through the decision-making scores. Different building functional typologies perform different energy consumption patterns in terms of activity types, occupied hours, internal gains, etc. The more detailed the sub-categories that buildings are divided into, the more accurate will be the analyses in terms of energy. In this study, a social housing archetype which has a 4 apartment/floor plan scheme was designated. Firstly, the independent variables were determined for the parameterization process. The main independent variable was designated as the thermal insulation material used and its thickness, since the aim is the selection of the optimum thermal insulation material. The dependent variables are identified as energy, cost, thermal comfort, and environmental impact performance. Energy consumption is calculated through the use of the Energy Plus simulation tool. Such consumption is based on an hourly basis, with 6 time steps per hour. Using this approach, the life cycle cost of the related components were calculated with a modified global cost formula as the sum of the initial investment cost, the net present value annual cost of the energy costs, transposed with the development rate of energy. In this approach, indoor environmental thermal comfort is evaluated based on the EN 15251 Adaptive comfort model, and evaluated within Category III. The influence of the material selection on the environmental impact performance of the building is calculated through the embodied carbon (kgCO2e/kg) and the embodied energy (Mje/kg) of the materials used. In the proposed approach, all scenarios occurring by parameterization are calculated in terms of life cycle cost and primary energy consumption through a 30 year life span. The cost optimal point within the whole alternatives is obtained, and alternatives beyond the cost optimal level are listed for the purposes of multiple criteria decision-making. Alternatives were listed with the associated decision-making criteria for high performance building design in the form of investment cost, primary energy consumption, thermal comfort, and environmental impact values. In order to be compared with each other, all criteria values were normalized. Furthermore, the normalized values of the criteria were summed in order to obtain the total score of the alternatives. In this step, the weighting factors of each criterion were considered either as being equal or as the calculated weightings based on the AHP pairwise comparison method for criteria. Finally, best alternatives were determined for the decision maker, in order to develop a high performance building design. The analysis show that the performance criteria that have been evaluated have an important impact on the alternatives and the results. Thus, it is meaningful to include the four determined performance criteria (energy, cost, thermal comfort, and environmental impact) in the decision-making process in order to select the best alternatives. The results show that proper decision-making on the selection of thermal insulation building materials, with different environmental and economic attributes, ensure a higher performance with regard to buildings. The proposed approach influences the selection of the best alternative, which allows the personal preferences of the decision maker to be met, without digressing from the scope of EPBD. It is clear that the methodology used in multiple criteria decision-making, and the weighting factors of the criteria, are quite substantial and effective in terms of the results of the decision-making process. The study should be improved by assigning different weighting factors to decision-making criteria, in order to identify the deviations resulting from the different weighting factors. Thus, further studies should be conducted incorporating different decision-making methods, and variations in terms of the criteria and the weighting factors should be analyzed. It is also possible that a tool can be generated by using the proposed methodology, to offer a quick analysis to meet further demands and for the design processes of different archetypes.