Integrated Energy and Environmental Life Cycle Assessment of Office Building Envelopes
Rahman Azari1
1
Assistant Professor, College of Architecture, University of Texas at San Antonio, 501 W. Cesar E.
Chavez Blvd., San Antonio, TX 78207, US; Tel:(+1)210.458.3010;
[email protected]
Abstract Building envelope, which separates the interior conditioned from exterior unconditioned environment of a building, is the key determinant of thermal and energy performance in many types of buildings. The building envelope is primarily designed to restrict the heat transfer between inside and outside in order to regulate the thermal characteristics of the interior environment and reduce the heating, cooling and electric lighting demand of buildings. The key goal of the present research is to examine the life cycle energy and environmental performance of building envelopes by conducting a comparative energy and environmental life cycle assessment (LCA) study of several envelope scenarios in which some of the major components of building envelope vary. The varying components include insulation material, window-to-wall ratio (WWR), window frame material, and double-glazing cavity gas. The generic building model used in this study was a hypothetical 2-story office building with 335 square meters (3600 square feet) of floor area. The results revealed scenarios with low to medium WWR and fiberglass window frame result in the lowest impacts. The research also
shows that use phase of the life cycle is the primary contributor to most environmental impact categories for all scenarios. Keywords: life cycle assessment (LCA), building envelope, glass curtain wall, cavity wall 1. Introduction Building envelope plays a significant role in buildings’ performance and literature suggests that the improvement of envelope can reduce energy costs, healthcare expenses, and mortality rates, and would significantly enhance comfort and quality of life of millions of people [1]. The statistics indicate that space heating and cooling constitute 37% of primary energy consumption in the US building sector, which is primarily caused by the building envelope characteristics, such as window/wall conductivity and infiltration rate [2]. Envelope is also the largest building component and can play a major role in how buildings affect the environment [3].
2. LITERATURE REVIEW There exist numerous studies on LCA of building envelopes. Kim [4] is among few studies that conduct an integrated energy (using eQuest 3.6 as the tool) and LCA (using SimaPro 7.1) analysis. The study examines how much difference in energy and environmental burden would result from the change from a glass curtain wall system to a transparent composite façade system (TCFS) over their entire life cycle. It concluded that the life cycle energy use associated with TCFS system and the equivalent CO2 emissions associated with that were 93% and 89% of that of the glass curtain wall system, respectively. The study also shows that the environmental performance of the TCFS system can further be improved by enhancing the system’s energy performance and durability, and recycling the materials at the end of their life-cycle [4].
In another attempt, Stazi et al. [5] apply an integrated energy-LCA analysis and focus on pre-use and use phases of a solar/Trombe wall. To find the appropriate solar wall design solution with optimized energy and environmental life cycle performance, their study examined the impact of changes in wall materials, wall thickness, glazing type, and frame material. Their parametric analysis entailed measuring the impact on energy use and the environment of one design parameter at a time and then using a “full level factorial plan technique” to find the optimal configuration of design parameters. A conclusion of this study is that the building envelope consisting of a 20-cm thick aerated concrete wall with double-glazed wood-framed window leads to the lowest global warming potential (GWP) among the 16 configurations studied [5]. Ottelé et al [6] did a comparative LCA study on a regular non-vegetated brick facade, a brick facade covered (directly or indirectly) with vegetation, and a living wall system (LWS) in the context of the Netherlands. While many of the LCA studies only address GWP as the impact category, this study also assessed the impacts associated with human toxicity and fresh water aquatic ecotoxicity. The study, which additionally addressed the energy aspects, concluded that, while the LWS wall represent relatively low environmental impacts compared with other systems, greening the building envelope does not necessarily result in lower environmental damages. Conversely, the study shows that brick walls covered with vegetation installed on a steel structure can have higher environmental impacts [6]. Salazar and Sowlati [7] focused on window systems and applied a variety of data sources as well as SimaPro LCA tool and IMPACT 2002+ to study the life cycle environmental impacts associated with the use of residential windows with various frame materials including PVC, aluminum-clad wood and fiberglass. The study concluded that the window with PVC as frame material makes the largest damage to the environment with respect to global warming,
respiratory inorganics, and acidification. Similar to Salazar and Sowlati [7], there exists a variety of literature that focus on the life cycle impacts of windows [8,9]. The review of literature in the field shows that there is a wealth of studies in the field that investigate energy performance of the building envelopes. Few studies, however, attempt to study both energy and life cycle environmental performance through an integrated framework. Also, there exist few parametric studies in the area of building envelopes’ environmental impacts. Another gap in literature has to do with the environmental impact categories as few of the impact categories receive the most attention. The key objective of the present research was to assess and compare the energy use and environmental impacts associated with six scenarios of building envelope in which the envelope’s wall-to-window ratio (WWR), insulation material, double-glazing cavity gas, and window frame material vary. To achieve the research objective, an integrated energy and environmental LCA analysis was conducted.
3. RESEARCH METHODS Environmental LCA is a strong quantitative method of analysis that is used to quantify and evaluate the impacts on environment caused by products and processes. There are three major techniques of LCA. In environmental process-based LCA, the most widely-applied method in the field of built environment, environmental inputs (materials, energy) and outputs (waste, emissions) associated with each phase in a given product’s life cycle are identified, quantified, aggregated for the entire life cycle, and then classified based on their impacts [10]. They are then translated into indicators representing various categories of environmental impacts [10]. The Economic Input-Output (EIO) LCA technique is based on the US economy models and links monetary value of industry sectors to their environmental outputs [11]. The third technique,
Ecologically-based LCA (E-LCA) [12] is based on an “integrated ecological-economic model of the US economy” and additionally addresses the role of eco-system services (biogeochemical cycles, disease regulations, etc.). The present study relied on Athena Impact Estimator (IE), an environmental LCA tool that uses a process-based LCA technique. Figure 1 illustrates the LCA methodology followed by this study based on ISO 14040 [13]. The LCA methodology consists of four phases of goal and scope definition, inventory modeling and analysis, impact assessment, and interpretation of results. The methodology is explained in detail at the following sections.
Figure 1. Energy-LCA Methodology
3.1. Goal and Scope Definition The first phase of the LCA, goal and scope definition, deals with defining the audience and application of the LCA, its functional unit, and system boundary and setting the impact
categories of interest. The present study targets built environment researchers and practitioners and aims to show them how the change in building envelope characteristics can affect its life cycle environmental impacts. It also intends to present an example of how integrated energy and LCA analysis can be used early on in the design process. Functional unit is defined as “quantified performance of a product system for use as a reference unit” [13]. In other words, it is the identical function to be performed by all the building envelope scenarios in order to make the comparison meaningful. The functional unit for the present study was defined to enclose a hypothetical two-story office building in Seattle, US, with 335 square meters (3600 square-feet) of floor area and the service life of 60 years. In addition, the same total R-value, hence U-factor, was assumed for the opaque part of all envelope scenarios. A reason for choosing a hypothetical building case, rather than an actual case, was similarity of this situation to early design process situations where a great deal of uncertainty is involved. The hope was to show that it would still be possible to conduct energy performance and LCA analyses even with limited design certainty of early design process. Table 1 shows the components of building envelope (wall and window) and table 2 shows the six envelope scenarios of interest in this study, which were chosen out of all possible combinations of envelope design parameters, along with their varying components. While the chosen envelope components and their alternatives were selected to approach the general new construction practice in North America, it is important to note that the alternatives chosen for design parameters, especially for WWR, may not reflect compliance with local energy codes. For example, while the local energy code allows a maximum gross WWR of up to 40% (for buildings with daylight control), the present study considered three net WWR alternatives of 40%, 60%, and 80% for the entire building. Obviously, two of the WWR alternatives (60%, 80%) do not meet the energy code requirements; however the code requirements consider only
the energy implications of design parameters while this study aimed at examining both energy and environmental impacts associated with the changes in building envelope parameters. Therefore, the researcher chose to be open-minded about the issue and consider a variety of alternatives in this exploration, regardless of their compliance with local energy codes, as this had the potential to provide insight into an angle – i.e. environmental impacts - that energy codes do not comprehensively view. That said, however, the R-value of the opaque part of the envelope was chosen to be constant across the scenarios and to meet the local code requirements; just to avoid having too many variables in the study and the subsequent complication in interpreting the results. Table 1. Components of building envelope Building Envelope Brick wall Split-faced brick veneer – 10 cm (4”) Air gap – 10 cm (4”) Insulation Fiberglass Batt 2 2 (R=3.52 K.m /W = 20 h.ft .°F/Btu) Mineral Wool Batt 2 2 (R=3.52 K.m /W = 20 h.ft .°F/Btu) Vapor barrier (polyethylene) Concrete brick – 10 cm (4”) Gypsum plasterboard – 1.25 cm (0.5”)
Window system Window frame Aluminum PVC Fiberglass Glazing Low-e double-glazing (LE DG) with air Low-e double-glazing (LE DG) with argon
Table 2. Scenarios of building envelope used for the study
*
Scenario
WWR*
Insulation
Glazing
Frame material
SC1 SC2 SC3 SC4 SC5 SC6
40% 40% 60% 60% 80% 80%
Fiberglass Batt R20 Mineral Wool Batt R20 Fiberglass Batt R20 Mineral Wool Batt R20 Fiberglass Batt R20 Mineral Wool Batt R20
Air-filled LE DG Argon-filled LE DG Air-filled LE DG Argon-filled LE DG Air-filled LE DG Argon-filled LE DG
Aluminum Fiberglass Fiberglass PVC PVC Aluminum
WWR values are net window to wall ratios. WWR values of 60% and 80% for the entire building do not meet the Seattle energy code. However, as explained in the article, the researcher decided to still consider these two alternatives too.
System boundary is defined as a “set of criteria specifying which unit processes are part of a product system” [13]. Through the system boundary, the LCA researcher decides to include in the LCA study, or exclude from it, certain stages of the life cycle of the product of interest. System boundary also shows which environmental inputs or outputs are included in the research. Figure 2 shows the system boundary of the present LCA study. The environmental impact categories of interest included fossil fuel consumption, global warming, acidification, eutrophication, ozone depletion and smog formation. Table 3 shows a summary of scope definition and major assumptions of the present study.
Figure 2. LCA’s System boundary Table 3. Summary of scope definition and major assumptions Functional unit System boundary Impact categories Technique Tools
Major assumptions
Building envelope to enclose a hypothetical two-story office building in Seattle, US, with 3600-square-foot floor area and the service life of 60 years • Pre-use, use, and end of life stages of life cycle and associated transportation • Environmental inputs: natural resources, electricity and energy • Environmental outputs: air and water emissions, waste Fossil fuel consumption, global warming, acidification, eutrophication, ozone depletion, smog formation • Environmental process-based LCA • Athena IE for life cycle inventory modeling and impact assessment • eQuest 3.65 for energy performance simulation • The same R-value and U-factor for the opaque part of envelope is assumed across scenarios. • The WWR values are based on net window to wall ratios. • The inventory data used in Athena IE assumes [14,15,16]: ü 50% aluminum recycled in production of aluminum window frame, 14% external cullet (recycled glass) in glass production, 98% structural members recycled, 15% gypsum board recycled, 55% brick recycled, and 100% of other materials landfilled. ü 25% waste of mortar in brick envelope construction, 5% waste of fiberglass insulation.
3.2. Life Cycle Inventory Modeling Life Cycle Inventory (LCI) modeling is the second phase of LCA at which the environmental inputs and outputs associated with the envelope scenarios are identified and quantified, taking into account the functional unit of the LCA study, system boundary and the major assumptions, as specified in table 3. Athena Impact Estimator for Buildings (Athena IE) [17] was used in this research for the inventory modeling/analysis and impact assessment. Athena IE is a cradle-to-grave LCA tool capable of addressing the entire life cycle of a building from raw material extraction through manufacturing, transportation, construction, maintenance, repair and replacement, to demolition and disposal. The tool relies on a variety of manufacturer data, US life cycle inventory (US LCI) database, Ecoinvent, its own LCI database, and other data sources to model and analyze the environmental performance of buildings [16]. The LCI results include raw materials, emissions to air and water, and the waste generated through demolition stage of life cycle. To account for operational energy consumption, however, the tool needs to be fed by the outcome of energy simulation tools.
Material inputs: The six scenarios of building envelope were first modeled in the userfriendly interface of Athena IE. Roof, floors, foundations, and other components of the hypothetical building case were assumed to be of identical material, and with identical quantities, across the scenarios and therefore were not modeled in the tool as their presence in the model could not affect the comparative purpose of this research. Table 4 shows the material inputs for the six scenarios of building envelope, as developed by Athena IE, as part of the inventory modeling of the present research.
Table 4. Material inputs and their quantities in kg for the six scenarios Service Life (years) [14,15]
Gypsum plasterboard (1/2”) Polyethylene (vapor barrier) Concrete Brick Fiber Glass Batt Mineral Wool Batt Split-faced brick Mod. Bitumen membrane Galvanized sheet Galvanized stud Cold rolled sheet Mortar Joint compound Nails Paper tape Aluminum frame PVC frame Fiberglass frame LE DG with air LE DG with argon Screws, nuts and bolts Total weight
*
60 NR 60 NR NR 60 NR NR NR NR NR NR NR NR 20-25 17-19 NR 10-30 10-30 NR
SC1
SC2
SC3
SC4
SC5
SC6
3270.2 21.1 122634 147.5 29264.9 1304.9 746.2 1173.0 107.2 38370.9 291.4 19.1 3.3 648.5 6811.4 31.1 204844.7
3270.2 21.1 122634 723.6 29264.9 1304.9 746.2 1173.0 107.2 38370.9 291.4 19.1 3.3 1003.5 5222.1 31.1 204186.5
2180.2 14.1 81756 98.3 19510.0 869.9 746.2 1181.4 71.5 25580.6 194.3 12.8 2.2 1229.0 7973.1 31.1 141450.7
2180.2 14.1 81756 482.4 19510.0 869.9 746.2 1181.4 71.5 25580.6 194.3 12.8 2.2 2113.4 7973.1 31.1 142719.2
1090.1 7.0 40878 49.2 9755.0 435.0 746.2 1188.4 35.7 12790.3 97.1 6.4 1.1 2440.3 10742.1 31.1 80293
1090.1 7.0 40878 241.2 9755.0 435.0 746.2 1188.4 35.7 12790.3 97.1 6.4 1.1 917.1 14011.4 31.1 82231.1
NR= Not Reported by Athena IE
Operational Energy inputs: A natural result of variation in building envelope design parameters is the change in envelope’s thermal performance, and therefore, building’s operational energy use. In other words, the choice of insulation material, WWR, and fenestration system not only affects resource consumption, embodied energy and impacts associated with the envelope but also the energy that is used during occupancy phase of building for heating, cooling, lighting, ventilation, etc. This energy is called operational energy referring to its application for the operation of building. While the functional unit of study assumed the same total R-value and U-factor for the opaque part of the envelope – implying that insulation materials in this LCA study were chosen with different thicknesses in order to have the same R-value across the scenarios -, other envelope design parameters such as WWR, cavity gas and window frame material, still needed to
be examined with respect to their impacts on operational energy use. Athena IE, like other LCA tools, does not perform operational energy simulation; therefore, to address the change in thermal and energy performance, it was necessary to simulate the performance of the building case under all six scenarios using energy simulation software. The results of operational energy analysis then could feed Athena IE for further impact assessment analysis. eQuest 3.65 [18] was used for energy simulation. The tool, which relies on DOE-2 as simulation engine, uses hourly weather data to predict hourly energy use of buildings. Using eQuest 3.65 as building performance software, the energy performance of each envelope scenario was simulated for the climate zone of Seattle, US (climate zone 4C). The results of the energy analysis by eQuest 3.65 were then manually added as data entry to the Athena IE LCI modeling. Figure 3 shows the results of energy analysis with the breakdown of the end-use.
Figure 3. Annual operational energy consumption of the scenarios as predicted by eQuest 3.65
3.3. Impact Assessment Impact assessment was the next step of the study. Impact assessment phase of the LCA deals with classifying the environmental inputs and outputs of the scenarios based on their impacts on environment and aggregating them into indicators representing those impacts.
Classification and characterization are two major steps in impact assessment. In classification, the environmental input and output results of the LCI modeling are assigned to appropriate impact category [10]. In characterization, the quantity of the each environmental input/output is multiplied by its corresponding characterization factor for that impact category to convert the input/output quantity to an environmental measure representing the environmental category of interest. For instance, methane and CO2 both contribute to global warming, and the quantity of methane emissions should be multiplied by its corresponding characterization factor to yield the methane contribution to global warming as expressed in kg of equivalent CO2. Athena IE, which relies on US EPA TRACI methodology [19], was used for the impact assessment of the present study. The impacts of the six scenarios were assessed in six impact categories of fossil fuel consumption, global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), ozone depletion potential (ODP), and smog formation potential (SFP). The impact results generated by Athena IE entail product, construction, use, and end of life (disposal) phases of building envelope life cycle as well as the transportation share associated with each phase. For the sake of this research, the product and construction phase impacts were aggregated into ‘pre-use’ phase, and the results are reported for pre-use, use, and end-of-life phases of life cycle.
4. Results and Discussions Tables 5 and figure 4 show the summary results of the impact assessment phase. Table 6 present the ranking of scenarios based on their energy use and environmental impacts. The results are explained in detail in the following sections.
Table 5. Energy and environmental impacts of scenarios
Scenario
Operational Energy
SC1 SC2 SC3 SC4 SC5 SC6
(MJ) 148200 139457 145565 149570 151573 152387
Fossil Fuel
GWP
AP
EP
ODP
SFP
(MJ)
(kg CO2-eq)
(kg SO2-eq)
(kg N-eq)
(kg CFC11-eq)
(kg O3-eq)
7564782 6949790 7125748 7382055 7255951 7402221
503097 467091 478393 493976 488726 498966
3917 3615 3739 3863 3841 3928
72 60 69 74 81 100
0.000822 0.000702 0.000881 0.000935 0.001123 0.001346
10305 9533 9303 9593 8985 9419
Table 6. Ranking of scenarios based on their energy use and environmental impacts (1=lowest; 6=highest) Scenario
Weight
Operational Energy
Fossil Fuel
GWP
AP
EP
ODP
SFP
SC1 SC2 SC3 SC4 SC5 SC6
6 5 4 3 1 2
3 1 2 4 5 6
6 1 2 4 3 5
6 1 2 4 3 5
5 1 2 4 3 6
3 1 2 4 5 6
2 1 3 4 5 6
6 4 2 5 1 3
4.1.Fossil Fuel and operational energy Fossil fuel category addresses the equivalent megajouls of fossil fuel that is consumed over the life cycle of each scenario. The results of fossil fuel consumption, as shown in tables 5 and 6 and figure 4, reveals that the building envelope scenario 2 (SC2) consumes the least fossil fuel, among the six scenarios, followed by SC3. SC2 envelope scenario entails the use of mineral wool batt insulation and argon-filled fiberglass-framed low-e double-glazed (DG) window on a 40% WWR envelope. SC3 scenario involves fiberglass batt insulation, air-filled fiberglassframed low-e DG with 60%WWR. On the other hand, SC1 and SC6 scenarios result in the greatest consumption of fossil fuel, respectively. This high life cycle fossil fuel energy use in SC1 and SC6 can be attributed to the use of aluminum, an energy intensive material, as the window frame.
Figure 4. Environmental Impacts of scenarios.
A relatively similar pattern exists in operational energy analysis results, as shown in figure 3 and tables 5 and 6. SC2 and SC3 result in the lowest consumption of energy during occupancy phase of the building’s life cycle while SC6 and SC5, respectively, lead to the highest energy use. Both SC6 and SC5 scenarios have to do with 80% WWR. Energy performance analysis also indicate that 52-56% of operational energy in all scenarios is consumed to heat, cool and illuminate the building. Considering that the scenarios
were chosen to have similar total R-value and U-factor for the opaque envelope (even though they were assumed to be made from different insulation materials) and that they all use low-e DG glazing, the variation in heating and cooling energy use across scenarios could be attributed to their variation of WWR, cavity gas (air/argon) between glass panes, and the frame materials. 80% WWR in SC5 and SC6 has resulted in the lowest share of heating energy use (31%) in overall energy use and the highest share of cooling energy use (12%) across the scenarios. SC1 with 40% WWR, on the other hand, entails the highest share of heating energy use (38%) and the lowest share of cooling energy use (7%), across scenarios. Within the scenarios having the same WWR, one would expect for the argon-filled low-e DG scenario to result in lower operational energy use than the scenario with air-filled low-e DG just because of the improved thermal performance of argon-filled windows; however, the energy analysis results, as shown in figure 3, reveal that this is not always the case and the frame material also plays a role. 4.2. Global Warming Potential (GWP) Global warming is caused by the contribution of greenhouse gases and is measured in kg of equivalent CO2. Because a major source of contribution to global warming is the emissions caused by the combustion of fossil fuels, the same GWP ranking of scenarios as fossil fuel consumption ranking could be justified. The low GWP in SC2 and SC3, compared to other scenarios, has to do with the use of fiberglass as window frame. This is in line with literature suggestions about the low environmental impacts of fiberglass window frames [7]. Moreover, both SC1 and SC6 scenarios use energy-intensive aluminum for window frame and result in the highest global warming impacts.
4.3.Acidification Potential (AP) and Eutrophication Potential (EP) Acidification has to do with transformation of air pollutants into acids, which decrease the pH-value of rainwater [20]. AP is about kg of equivalent sulfur dioxide (SO2) that a certain product contributes to acidification over its life cycle. Eutrophication Potential (EP), measured in kg N-eq, addresses the contribution to growth of algae in water bodies which results in blockage of sunlight [20]. In both impact categories, the lowest AP and EP are caused by SC2 and SC3 while SC6 contribute the most to both. About 5% to 6.8% of acidification potential across scenarios have to do with the pre-use phase while 93%-95% of the AP is caused by the use phase. In eutrophication category, 5%-12% of the impacts are represented by pre-use phase and 87.5% to 94.6% of EP is caused as a result of the use phase in the scenarios’ life cycles. End-of-life phase has a negligible share in both AP and EP, similar to other impact categories. 4.4. Ozone Depletion Potential (ODP) Man-made ozone depletion is mainly caused by the release of CFCs. While CFCs were phased out through the Montreal Protocol, there are emissions other than CFCs, such as HCFCs, too that can lead to ozone depletion. The results of impact assessment in ozone depletion category, as shown in tables 5 and 6 and figure 4, present the SC2 as having the lowest ozone depletion impact and SC6 and SC5 as having the highest impacts. In contrast to other environmental impact categories where the pre-use phase of the life cycle had a small share in contribution to overall impacts of the scenarios, this phase has a significantly larger share in ODP category. Indeed, the pre-use phase contributes between 41% to 64% to ODP across the scenarios. The high share of pre-use phase in ODP has to do with the use of CFCs or HCFCs in insulating manufacturing. The contribution of the use phase in ODP ranges from approximately 35% (SC1) to 58.4% (SC6).
4.5.Smog Formation Potential (SFP) Smog formation takes places as a result of reactions with sunlight of air emissions trapped near the earth, which results in damages to environmental damages as well as damages to human health [21]. SC5 and SC3 lead to the lowest SFP, respectively, while SC1 contributes the most to this environmental impact category. 19% to 30% of the contribution to SFP across scenarios is caused by pre-use phase. Use phase, on the other hand, entails 67% to 80% of contribution to SFP.
5. CONCLUSION A limited number of building envelope scenarios for a hypothetical typical cuboid-shaped office building case were investigated with respect to their energy use and life cycle environmental impacts. The scenarios represented various window-to-wall ratios, insulation materials (with different thicknesses to have the same R-value), double-glazing window cavity gases, and window frame materials. Although some of the alternatives of envelope design parameters, especially those of WWR, did not meet the local energy code requirements, their presence in the study was still considered to be important because of their significant effect on building envelope’s environmental impacts; an area that energy codes do not touch in a comprehensive manner. Therefore, it is important to interpret the results of present research in the light of its assumptions and limitations. The ranking of scenarios based on their environmental impacts reveals that the lowest environmental impacts in all categories of interest, except smog formation, are caused by the use of SC2 (40% WWR, mineral wool insulation, fiberglass-framed argon-filled low-e DG), followed by SC3 (60% WWR, fiberglass batt insulation, fiberglass-framed air-filled low-e DG). Both of these scenarios entail the use of low to average WWR and fiberglass window frame material. Of these two scenarios, only SC2 meets the local energy code; however, it is important
to note that, based on the local energy code requirements, it is possible for parts of a building (for instance, a certain façade) to have WWR values of greater than 40%, if other parts compensate that by having lower WWR values. Among the scenarios, SC5 is the scenario that represents the lowest smog formation potential. On the other hand, SC6 represents the highest environmental impacts in most categories, including operational energy use, acidification, eutrophication, and ozone depletion categories. It also contributes as the second highest scenario to fossil fuel consumption and global warming. The SC6 scenario entails 80% WWR, mineral wool insulation, and aluminum-framed argonfilled low-e DG. The SC1 scenario, with 40% WWR, fiberglass batt insulation, and aluminumframed air-filled low-e DG, includes the highest contribution to fossil fuel consumption, global warming potential, and smog formation potential. The environmental impacts associated with aluminum production could be an explanation for the high impacts of both SC6 and SC1. In almost all categories of environmental impacts, the use phase, which entails operation and maintenance stages of the life cycle, represents the primary contributor to overall life cycle impacts of the scenarios. For fossil fuel consumption, global warming, acidification and eutrophication categories, the contribution of use phase is more than 90%. In ozone depletion category, the contribution was 35-58% and in smog formation, it was at least 67%. This highlights the importance of occupancy phase of the building and the need to reduce the impacts in this phase. An implication of this is the significant need to implement high-performance energy-efficient strategies in buildings – to reduce operational energy use – and to apply durable materials with longer life spans – to reduce the replacement and maintenance needs. In addition to presenting its results, this study represents an example of how an integrated energy and environmental analysis can inform design decisions in the field of built environment early on in the design process.
5.1. Limitations and Recommendations There were several limitations associated with the present research. The first limitation has to do with the scope of research. Due to resource limitations, the study focused on a limited number of envelope design parameters; and only a limited number of design parameter combinations, out of all possible combinations, were examined. Moreover, some of these scenarios do not meet the local energy code requirements and were included in the research due to their importance in affecting the environmental impacts. Future research is recommended to conduct a parametric analysis using more combinations and scenarios of building envelopes. This parametric study should take into account all variables affecting the energy and environmental performance of building envelopes, including geographical location and climate zone, building type, orientation, glazing’s solar heat gain coefficient (SHGC), etc. Furthermore, optimization algorithms could be implemented to optimize the outcome of such parametric study. The second limitation has to do with the available LCA analysis tools. Currently, there is no comprehensive environmental LCA analysis tool capable of doing building energy performance analysis. Conducting separate energy and environmental analyses and feeding the LCA tool with energy analysis results may introduce bias to LCA results because of occasional discrepancies in the material databases across the tools. The industry needs to develop a comprehensive building performance tool that uses BIM models, for instance, as the input to LCA analysis; all within one software framework. In addition, Athena IE which was used as the tool for LCI modeling and impact assessment has a limited database of materials which made the addition of some other scenarios impossible.
The third limitation has to do with the fact that the present study ignores the cost and social aspects of life cycle sustainability. Future research can especially focus on an integrated life-cycle cost and environmental analysis of the building envelope scenarios. References [1] IEA. Technology roadmap: Energy-efficient building envelopes. International Energy Agency, 2013. [2] DOE. Windows and building envelope research and development: roadmap for emerging technologies, US Department of Agency, 2014. [3] G. Manioglu, Z. Yilmaz, Economic evaluation of the building envelope and operation period of heating system in terms of thermal comfort, Energy and Buildings 38(3) (2006) 266-272. [4] K. Kim, A comparative life cycle assessment of a transparent composite facade system and a glass curtain wall system, Energy and Buildings (2011) 3436-3445. [5] F. Stazi, M. Alessio, P. Munafo, Life-cycle assessment approach for the optimization of sustainable building envelopes: An application on solar wall systems, Building and Environment 58 (2012) 278-288. [6] M. Ottelé, K. Perini, A.L.A. Fraaij, E.M. Haasa, R. Raiteri, Comparative life cycle analysis for green facades and living wall systems, Energy and Buildings 43 (2011) 3419–3429. [7] J. Salazar T. Sowlati, Life cycle assessment of windows for the North American residential market: case study, Scandinavian Journal of Forest Research 23(2) (2008) 121-132. [8] S. Citherlet, F. Di Guglielmo, J.B. Gay, Window and advanced glazing systems life cycle assessment. Energy and Buildings 32 (2000) 225-234. [9] E. Syrrakou, S. Papaefthimiou, P. Yianoulis, Environmental assessment of electrochromic glazing production, Solar Energy Materials & Solar Cells 85 (2005) 205-240. [10] R. Heijungs, S. Suh, Computational Structure of Life Cycle Assessment. Kluwer Academic
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[21] American Institute of Architects (AIA), AIA Guide to Building Life Cycle Assessment in Practice, 2010.
The author has requested enhancement of the downloaded file. All in-text references underlined in blue are linked to publications on ResearchGate.
