To conduct a LCA study, ISO 14, 040 series standards should be complied with. The ISO 14, 040 series standards for the LCA address quantitative methods for assessing the environmental aspects of a product or service in its entire life cycle stages. The ISO 14, 040 is an overarching standard encompassing all four phases of the LCA (ISO, 2006), including: 1) ISO 14, 041 deals with defining the goal and scope phase, and life cycle inventory methods; 2) ISO 14, 042 deals with life cycle impact assessment methods; and 3) ISO 14, 043 deals with life cycle interpretation methods. The relationship between these four phases stipulated in ISO 14, 040 series standards are illustrated in Fig. 1 (Margni and Curran, 2012).
Definition of goal and scope is the first step of the LCA. In this step, the Functional Equivalent, system boundaries, and quality criteria for inventory data will be defined (Sharma et al., 2011). The goal and scope should be defined clearly enough to make sure that the breath, depth, and detail of the study are addressed sufficiently (ISO, 2006).
Inventory analysis is the second step of the LCA. It includes data collection and calculation procedures (ISO, 2006). Life cycle inventory (LCI) is the data collection portion of the LCA (Athena Sustainable Materials Institute, 2018b). It tracks material and energy flows in and out of the product system. The elements of inventory include raw materials, energy by type, water use, and emissions to air, water, and land by specific substance. The process of analysis can be very complex and may involve many individual unit processes in one supply chain, as well as tracking different substances for hundreds of times (Athena Sustainable Materials Institute, 2018b).
Life cycle impact assessment (LCIA) is the third step of the LCA, which is used to make connections between the inventory of basic flows for the system of the product and its potential environmental impacts (Hauschild and Huijbregts, 2015). In the LCIA, different environmental impacts are assigned to different environmental impact categories (Sharma et al., 2011). The environmental impact categories refer to abiotic and biotic resource depletion, global warming, ozone depletion, etc. Globally, there are various calculating methods of environmental impact. In North America, Tool for Reduction and Assessment of Chemicals and Other Environmental Impacts (TRACI) methods are normally used. These methods focus on the following impact categories: ozone depletion, climate change, acidification, eutrophication, smog formation, and non-renewable energy consumption (Bare et al., 2012). The life cycle impact of the flows to and from the environment will be categorized and characterized by those methods. Complications may arise during the comparability of different LCA studies. Variables that may affect the LCIA include the system boundary, the functional equivalent, and specific LCIA methods chosen. When comparing two LCA studies, these factors are important to understand if the comparison is feasible (Athena Sustainable Materials Institute, 2018b).
Life cycle interpretation is the last step of the LCA. Life cycle interpretation includes the identification of significant issues and the evaluation of results. It deals with the interpretation of results from both the life cycle inventory analysis and life cycle impact assessment (Sharma et al., 2011).
In this study, Athena Impact Estimator for Building (IE4B) was used as the research base for the ISO 14, 040 series. The IE4B is a Canadian software for conducting an LCA assessment on a building, which is an open source software (Athena Sustainable Materials Institute, 2018a). It is applicable towards any types of new construction, renovations, and additions projects in North America. It can model over 1200 structural and envelope assembly combinations and allows for quick and easy comparisons of multiple design options. The IE4B provides the inventory profile for a whole building over its entire life cycle, i.e., from product stage to construction process stage, to use stage and finally, to the end of life stage. Such an inventory includes the flows from and to nature, emissions by energy, and raw materials to air, water and land. Athena becomes a leader in environmental declarations, including pioneered environmental building declarations (EBDs) to produce whole building life cycle declarations with a real commitment to accountability and transparency in sustainable design for building owners.
Another important section of Athena IE4B is its database and environmental impact assessment tool. Athena IE4B has its own life cycle inventory called Athena database. The TRACI 2.1 (tool for reduction and assessment of chemicals and other environmental impacts) was selected as the environmental impact assessment tool. It provides characterization factors for the LCIA for the analysis results. The environmental factors include ozone depletion, climate change, acidification, eutrophication, smog formation, human health impacts, and ecotoxicity (Bare et al., 2012). The level of sensitivity of a given factor to the LCIA results are different from one another at any given situation.
The sensitivity study was conducted by using Athena IE4B. The John W. Olver Design Building of the University of Massachusetts (USA) was selected as the reference building. This building is the largest and most technologically advanced academic contemporary wood structure. It is also the first building in the USA. that uses a wood-concrete composite floor system (Building and Construction Technology, 2017). This building has already been assessed using the first environmental building declaration (EBD) for a U.S. building by Athena Sustainable Materials Institute and published by United States Department of Agriculture (USDA) Forest Service, Forest Products Laboratory (FPL) (Gu and Bergman, 2018).
In this study, the research was based on the data acquired from the open source report from USDA FPL and was permitted by the Project Leader. In order to conduct the study, all the materials of different units were converted into the same unit. Then, all the material density values as provided by Athena IE4B could be used to convert their mass to volume. The volume of each material used in the representative building is given in Table 1. Concrete and wood were the main building materials that were used in the representative building, taking up 32.0% and 31.1%, respectively (Table 1). Thus, concrete and wood were considered as variables in the sensitivity study. Since steel is a main structural building material in construction, steel materials were also considered in the sensitivity study. Other construction materials were not considered due to their low quantity, which did not, therefore, influence the total volume to a significant degree.
Material used Volume (m3) Percentage (%) Aggregate 320 4.5 Concrete 2264 32.0 Gypsum 107 1.5 Insulation 1816 25.7 Roofing 45 0.6 Steel 71 1.0 Wood 2199 31.1 Other materials 250 3.5 Total 7072 100 Source: Gu and Bergman, 2018.
Table 1. Materials inventory of representative building.
Two case studies were conducted in this study, and one corresponding to the percentage of wood volume and the other corre-sponding to the proportional percentage of wood volume in relation to steel and concrete. The data of the raw materials and annual operational energy used from each step were used as inputs in Athena IE4B. Meanwhile, the total environmental impact results were compared with the published results for the baseline building.
In Case 1, the volume of wood materials was changed, meanwhile the volumes of all other materials, such as steel and concrete, were kept unchanged. The changes were made by increasing (+) or decreasing (–) the volumetric percentages of wood materials, such as by: –20%, –10%, +10%, +20%, and +50%. After estimating the environmental impact for each plan, the results were compared with the results of the baseline building. Thus, the sensitivity level of each environmental indicator could be understood by changing the total volume of wood materials. Athena IE4B was run by imputing the data for the raw materials and annual operational energy used from each step.
In Case 2, the volume of wood materials was changed by –20%, –10%, +10%, +20%, and +50%, while the volumes of steel and concrete materials were also changed accordingly by +20%, +10%, –10%, –20%, and –50%. In this case, the total volume of building materials used could be considered the same. The volumetric changes in wood, concrete, and steel materials could be considered as an alternative analysis. The results were also compared with those of the baseline building. The volumes of wood, steel, and concrete were input into Athena IE4B by considering the raw materials and annual operational energy used from each step.
2.1. The LCA analysis
2.2. Research base
2.3. Sensitivity analysis
In Case 1, only the wood materials were volumetrically changed in the reference hybrid timber building, in which every envi-ronmental impact result was divided by baseline results. The results from Case 1 for each step are shown in Table 2 and Fig. 2. As illustrated in Fig. 2, the trends of data of Case 1 are very clear. All the environmental indicators no doubt continuously increased with increasing wood materials. Based on the slope of each curve, it can be observed that the most sensitive indicator in this case was stratospheric ozone depletion, followed by tropospheric ozone formation, acidification of land and water, eutrophication, depletion of non-renewable energy resources, and finally global warming potential as the least sensitive indicator.
Summary measure –20% –10% 0 10% 20% 50% Global warming potential 3.9E+06 4.0E+06 4.0E+06 4.1E+06 4.1E+06 4.2E+06 Stratospheric ozone depletion 4.6E–02 4.7E–02 4.9E–02 5.1E–02 5.3E–02 5.9E–02 Acidification of land and water 2.6E+04 2.6E+04 2.6E+04 2.7E+04 2.7E+04 2.9E+04 Eutrophication 2.0E+03 2.0E+03 2.1E+03 2.1E+03 2.1E+03 2.2E+03 Tropospheric ozone formation 4.2E+05 4.3E+05 4.4E+05 4.5E+05 4.6E+05 4.9E+05 Depletion of non-renewable energy resources 4.8E+07 4.8E+07 4.9E+07 5.0E+07 5.0E+07 5.2E+07
Table 2. Results for changing wood materials.
Figure 2. Sensitivity study for environmental impacts in relation to change in volumes of wood materials.
In Case 2, the volumetric percentage of each major building material (i.e., wood, steel or concrete) used in the reference building was changed proportionally, meanwhile, the total volume of building materials was kept the same. The environmental impact results from each step for the Case 2 are shown in Table 3. Every environmental impact result was divided by baseline results and presented in Fig. 3. In Fig. 3, the trends of data from the Case 2 are different from the Case 1. Simultaneously changing the volumes of wood, steel, and concrete materials demonstrated a decrease in the environmental effect results. Eutrophication became the most sensitive indicator in this study, followed by global warming potential, depletion of non-renewable energy resources, acidification of land and water, tropospheric ozone formation, and stratospheric ozone depletion as the least sensitive indicator.
Summary measure –20% –10% 0 10% 20% 50% Global warming potential 4.3E+06 4.1E+06 4.0E+06 3.9E+06 3.8E+06 3.3E+06 Stratospheric ozone depletion 5.0E–02 5.0E–02 4.9E–02 4.9E–02 4.9E–02 4.8E–02 Acidification of land and water 2.7E+04 2.6E+04 2.6E+04 2.6E+04 2.6E+04 2.5E+04 Eutrophication 2.3E+03 2.2E+03 2.1E+03 2.0E+03 1.9E+03 1.6E+03 Tropospheric ozone formation 4.5E+05 4.4E+05 4.4E+05 4.3E+05 4.3E+05 4.1E+05 Depletion of non-renewable energy resources 5.2E+07 4.9E+07 4.9E+07 4.8E+07 4.6E+07 4.2E+07
Table 3. Results for changing three main materials.
Figure 3. Sensitivity study of environmental impacts in relation to simultaneous changes in all three main building materials.
The environmental impact results were compared with the published results for the baseline building (Table 4). The data were converted to a logarithmic scale for a better comparison. It can clearly found that there are some differences in results between the data of this study and those published, suggesting the calculation errors from Athena IE4B between the authors' and the published results. This could be due to the fact Athena IE4B is a kind of "black box" software, the operational errors can not be amended.
Summary measure Author's baseline building result Published baseline building result Difference (%) Global warming potential 4.83E+06 4.61E+06 –4.40% Stratospheric ozone depletion 4.28E–02 8.53E–02 99.23% Acidification of land and water 2.80E+04 2.39E+04 –14.83% Eutrophication 2.51E+03 1.38E+03 –45.12% Tropospheric ozone formation 4.46E+05 3.82E+05 –14.38% Depletion of non-renewable energy resources 5.79E+07 5.65E+07 –2.47%
Table 4. Comparison of baseline building between author's and published results.
Fig. 4 compares the authors and published results, suggesting a strong difference of 99.23% can be observed for stratospheric ozone depletion. Meanwhile, the lowest difference of –2.47% can be observed for the depletion of non-renewable energy resources.