Life cycle assessment tool for building assemblies

Transcription

1 Life cycle assessment tool for building assemblies J. Carmody Center for Sustainable Building Research, University of Minnesota, Minneapolis, Minnesota, USA W. Trusty & J. Meil Athena Sustainable Materials Institute, Ottawa, Ontario, Canada M. Lucuik Morrison Hershfield, Toronto, Ontario, Canada It is widely recognized in the field of Green Buildings that Life Cycle Assessment (LCA) is a conceptually preferable method for determining the environmental effects of materials, rather than relying on singular material properties or attributes, such as recycled content or distances traveled after the point of manufacture. However, the LCA tools that are currently available are not widely utilized. This is exacerbated by the failure of popular green building rating systems to fully incorporate LCA. The Green Building Initiative is introducing LCA as a more prominent aspect of the material section in its Green Globes building assessment and rating system. In addition to developing an LCA tool for use within Green Globes, a generic version of the tool, Athena EcoCalculator, has been developed and made available free of charge so that LCA information can be easily incorporated into other guidelines and rating systems. 1 INTRODUCTION In the past 15 years, many international, national and regional sustainable building guidelines have been developed. In North America, the LEED Rating System (Leadership in Energy and Environmental Development) developed by the U.S. Green Building Council (USGBC) has emerged in recent years with a high level of visibility and increasing market acceptance. Recently Green Globes, used for many years in Canada, has been introduced into the US by the Green Building Initiative (GBI) as a sustainable building rating system and guide for commercial buildings. In the US, sustainable commercial building guidelines have also been developed at the state and regional levels in New York, Minnesota, Florida and elsewhere. In the residential sector, there are many green building programs and guidelines as well. The future of these rating systems is unknown but there appear to be several driving forces that will shape their evolution. These include an emphasis on performance outcomes such as global warming impact, the need for regional variations, the need for variations for different building types, the trend toward more requirements rather than point-based alternatives, and more focus on actual building performance during occupancy and operation. 2 LIFE CYCLE ASSESSMENT A key aspect of moving toward more performance-based outcomes in sustainable design is the use of Life Cycle Assessment (LCA) to determine the embodied environmental effects of materials, rather than relying on singular material properties, such as recycled content or distances traveled after the point of manufacture. However, the LCA tools that are currently available are not widely utilized by most stakeholders, including those designing, constructing, purchasing or occupying buildings. LCA is a methodology for assessing the environmental performance of a product over its full life cycle. Environmental performance is generally measured in terms of a wide range of poten-

2 tial effects, such as: fossil fuel depletion other non-renewable resource use water use global warming potential stratospheric ozone depletion ground level ozone (smog) creation nutrification/eutrophication of water bodies acidification and acid deposition (dry and wet) toxic releases to air, water and land All of these measures are indicators of the environmental loadings that can result from the manufacture, use and disposal of a product. The indicators do not directly address the ultimate human or ecosystem health effects, a much more difficult and uncertain task, but they do provide good measures of environmental performance, given that reducing any of these effects is a step in the right direction. 3 LCA TOOLS IN NORTH AMERICA Current LCA-based tools in North America include BEES 3.0, which is a product comparison tool including some brand-specific data, and the ATHENA Environmental Impact Estimator (EIE) for analysis of whole buildings and assemblies. BEES 3.0 is intended for use at the specification or procurement stages of the process. Weighting factors are used to generate overall environmental and economic scores. ATHENA EIE is for use at the conceptual design stage. A range of indicators without weighting are generated to show environmental effects of changes in shape, design or material make-up of a building. Recently, a new tool has been developed for use with the Green Globes environmental assessment and rating system for commercial buildings. With funding from the Green Building Initiative (GBI), the tool was created by Morrison Hershfield Consulting Engineers in association with the University of Minnesota's Center for Sustainable Building Research and the Athena Sustainable Materials Institute. Modeled on the Building Research Establishment s (BRE) Green Guide to Specification, which has been used in the U.K. for over a decade, it measures the global warming potential and other environmental impacts of more than 400 common building assemblies in low- and high-rise categories. This same approach has been proposed for the U.S. Green Building Council s (USGBC) LEED Rating System in a report from a working group of the ad hoc LCA into LEED initiative that was launched in September The LCA tool and the entire Green Globes rating system is currently going through the technical committee responsible for GBI s consensus process, which complies with American National Standards Institute (ANSI) standards. GBI has authorized the Athena Institute to make a generic version of this tool freely available for use by other green building organizations, government entities, trade associations and universities. Regional versions of the tool are being developed to better reflect life cycle impacts based on local conditions. One of these regional versions is incorporated into the Minnesota Sustainable Building Guidelines. GBI also intends for the new software to be used in a forthcoming online version of the National Association of Home Builders Model Green Home Building Guidelines. Manufacturers can contribute relevant data to the U.S. LCI Database Project (

3 4 DESCRIPTION OF THE NEW LCA TOOL 4.1 Determining Building Assemblies and Functional Equivalence The tool includes a list of common building assemblies for office, commercial or industrial buildings. These assemblies are broken into categories based on broad system type (exterior walls, interior walls, window systems, beams, columns, intermediate floors, and exterior roofs). One challenge in comparing groups of building assemblies is establishing functional equivalence. Developing these more detailed functional equivalence categories presented a number of issues. Ideally, all assemblies in a defined category should have identical performance characteristics. However, assemblies are often relied upon to perform several different roles within a building. For example, an exterior wall may serve purposes related to structure, thermal resistance, fire resistance, air and vapor control, sound control, and aesthetics. This presents a complication: different users of the tool may have different intended purposes for a given assembly. For example, a structural engineer may want to compare assemblies with similar structural performance characteristics, while an architect or building science engineer may want to compare assemblies with similar thermal performance characteristics. While it is possible to create a list of assemblies within a category with identical properties in one area, such as thermal resistance, the result would be a list where some assemblies have unrealistic components. For example, if all exterior wall assemblies must meet a certain thermal resistance, most walls would utilize levels of insulation that are atypical and unavailable. A third functional performance issue is the fact that some assemblies are relatively uncommon or impossible to utilize for specific building types. For example, it is unlikely that a highrise building will be constructed with wood based columns and beams, and it would be unrealistic to offer these assemblies as viable options for some building types. This is particularly relevant if the LCA data of this assembly affects the ultimate rating of other assemblies. A last issue relates to the effect of an LCA rating on other aspects of sustainability. In general, assemblies with higher amounts of materials tend to have larger material environmental impacts than those with lesser amounts. For example, a 2x6 insulated wood stud wall will have higher material environmental impacts than a 2x4 insulated wood stud wall, if all material types within the system are the same. However, with many assemblies, material impacts represent only part of the environmental effect associated with the assembly: Their use also has operational effects, which could be significantly larger than their material effects over a building s life cycle. This raises the question of whether an LCA rating tool should promote the use of low embodied effect systems, when these systems might be a poor choice if operational issues are included for consideration. To address some of these issues, the new LCA tool is divided into two separate parts: highrise and low-rise construction. Note that many building assemblies were included in both building types. Low-rise buildings were considered to be four stories or less. Assemblies are broken into broad categories based solely on their use. Categories include exterior walls, interior walls, window systems, beams and columns, intermediate floors, and roofs. Factors for thermal resistance are provided for exterior walls and roofs, but solely for information purposes (they are not used to further categorize assemblies). No other functional equivalence categories are included. 4.2 Environmental Impact Indicators The ATHENA Environmental Impact Estimator (EIE) was used to develop LCA information on each building assembly. LCA results were developed for the following indicators: primary energy (fossil fuel depletion), global warming potential, air and water pollution indices, and solid waste. Each of these environmental measures used by the ATHENA Environmental Impact Estimator (EIE) software is described below. Embodied primary energy is reported in Giga-joules (Gj). Embodied energy includes all nonrenewable energy, direct and indirect, used to transform or transport raw materials into products and buildings, including inherent energy contained in raw or feedstock materials that are also used as common energy sources. (For example, natural gas used as a raw material in the produc-

4 tion of various plastic (polymer) resins.) In addition, the model and measure captures the precombustion (indirect) energy use associated with processing, transporting, converting and delivering fuel and energy. Solid waste is reported on a mass basis in kilograms and includes all solid wastes generated during manufacturing, construction, replacement and demolition that are destined for landfills. No attempt has been made to further categorize emissions to land as either hazardous or nonhazardous. All other measures are indices requiring more explanation and interpretation. They have been developed because of the difficulty of using and interpreting detailed life cycle inventory results. For example, it takes considerable expertise to understand and appreciate the significance of the individual emissions to air and water. Both categories encompass a relatively large number of individual substances with varying environmental impacts. Global Warming Potential (GWP) is a reference measure. Carbon dioxide is the common reference standard for global warming or greenhouse gas effects. All other greenhouse gases are referred to as having a CO2 equivalence effect which is simply a multiple of the greenhouse potential (heat trapping capability) of carbon dioxide. This effect has a time horizon due to the atmospheric reactivity or stability of the various contributing gases over time. The International Panel on Climate Change (2001) 100-year time horizon figures have been used here as a basis for the equivalence index: CO2 Equivalent kg = CO2 kg + (CH4 kg x 23) + (N2O kg x 296) While greenhouse gas emissions are largely a function of energy combustion, some products also emit greenhouse gases during the processing of raw materials. Process emissions often go unaccounted for due to the complexity associated with modeling manufacturing process stages. One example where process CO2 emissions are significant is in the production of cement (calcination of limestone). Because Athena uses data developed using a detailed life cycle modeling approach, all relevant process emissions of greenhouse gases are included in the resultant global warming potential index. The air and water pollution measures are similarly intended to capture the pollution or human health effects of groups of substances emitted at various life cycle stages. In this case we used the commonly recognized and accepted critical volume method to estimate the volume of ambient air or water that would be required to dilute contaminants to acceptable levels, where acceptability is defined by the most stringent standards (i.e., drinking water standards). The Athena EIE software calculates and reports these critical volume measures based on the worst offender that is, the substance requiring the largest volume of air and water to achieve dilution to acceptable levels. The hypothesis is that the same volume of air or water can contain a number of pollutants. 4.3 Allocation of Points and Weighting Issues The new tool allows an unbiased comparison of material assemblies across a set of five environmental indicators: embodied primary energy, which stands as a proxy for fossil fuel use; global warming potential; toxic releases to air; toxic releases to water; and solid waste. In the generic version of the tool, the Athena EcoCalculator, the LCA impacts are directly presented with no weighting or point allocations (Figure 1). In the Green Globes version, the goal is to translate the LCA results into points for the rating system. Rather than combining LCA scores across the five impact categories using some type of weighting so that each assembly can be assigned a single score, Green Globes will attribute points to each assembly in each impact category. This approach serves a valuable educational function because it lets the design team more readily see where they are earning their points. Points are awarded for a singular assembly by comparing its performance across each indicator measure to all other assemblies within the functional equivalence category (e.g. exterior walls for low-rise buildings). Assemblies with better than average performance across an indicator measure receive points (on a sliding scale), and the overall points for a particular assembly represent the sum of the points for each indicator category. There is no explicit weighting between indicator categories.

5 5 EXAMPLE OF LCA APPLIED TO BUILDING ASSEMBLIES AND MATERIALS A simple assembly comparison using the ATHENA EIE illustrates how LCA can be used to make design decisions taking proper account of environmental performance measures throughout the life of the materials. In Table 1, a given area of window assembly is compared to three typical solid wall assemblies in terms of primary energy use (read fossil fuel use), global warming potential, solid waste, air pollution index, and water pollution index. Table 1 illustrates the extent to which reducing window area has a beneficial environmental impact looking at the materials aspect in isolation, however such a decision must be examined in a whole systems context. For example, window design choices can have direct effects on building energy use if electric lighting and related cooling energy use are reduced because windows permit sufficient daylight to enter a space. Of course, they can also affect energy consumption as a result of heat loss and radiant solar heat gain. Window design decisions also have potential LCA impacts by affecting the design of many related components and systems in a building. A high performance window designed to maximize daylighting might reduce the need for light fixtures and perimeter heating in a space, as well as help reduce the size of the mechanical system. Similarly, if a particular window is chosen with a high visible transmittance glazing to enhance daylighting, glare may be increased resulting in the need for exterior or interior shading systems with related material impacts. These systems implications should and can be taken into account in LCA tools, such as the ATHENA EIE, balancing any increased environmental impacts from the materials (additional glazing layers and coatings for example) against the avoided impacts from reduced operating energy use over the life of the building. Those kinds of impacts must, of course, be balanced against other functional, cost, human comfort and aesthetic criteria to optimize systems from all perspectives. 6 CONCLUSION The adoption of LCA tools into Green Globes, LEED, and other regional rating systems represents a major step forward in what will likely be an ongoing integration of LCA into the sustainable design process. Over time, this process should strengthen the link between rating system scores and actual environmental benefits. The ultimate goal is to model the environmental impacts of whole buildings, so that rating systems can abandon the checklist approach and rate buildings based on a comprehensive model of their environmental performance, similar to the way energy modeling is done today. The final LCA tool developed in this project allows an unbiased comparison of material assemblies across a set of environmental indicator categories. The tool will be continually updated as new building product data or new assemblies emerge in the market. In addition, an alternative compliance path involving whole building LCA will be explored and possibly incorporated into Green Globes and other rating systems.

6 Figure 1: Athena EcoCalculator Interface Table 1: Wall Assembly Comparison ASSEMBLY TYPE Primary Energy per SF (MJ) GWP per SF (kg) Solid Waste per SF (kg) Air Pollution Index Water Pollution Index Window system with aluminum frame Low- E silver, argon-filled glazing CIP Concrete, brick cladding rigid insulation, vapor barrier gypsum board, latex paint CIP Concrete, stucco cladding rigid insulation, vapor barrier gypsum board, latex paint Steel stud, stucco cladding gypsum sheathing, batt insulation vapor barrier, gypsum board, paint

7 REFERENCES Anderson, J., Shiers, D.E. & Sinclair, M The Green Guide to Specification (3 rd edition). Building Research Establishment. Horst, S.W. & Trusty, W.B Integrating LCA Tools in LEED: First Steps. Proceedings: USGBC Greenbuild International Conference & Expo, Pittsburgh. Trusty, W.B Standards vs. Recommended Practice: Separating Process and Prescriptive Measures from Building Performance. Presentation at ASTM Symposium on Common Ground, Consensus Building and Continual Improvement: Standards and Sustainable Building, Washington, DC. Trusty, W.B. & Horst, S Integrating LCA Tools in Green Building Rating Systems. Proceedings: USGBC Greenbuilding International Conference & Expo, Austin, November pp Published in The Austin Papers: Best of the 2002 International Green Building Conference, Compiled by the Editors of Environmental Building News, Published by BuildingGreen, Inc., 2002, pp RESOURCES Athena Environmental Impact Estimator Athena Sustainable Materials Institute BEES 3.0 (Building for Environmental and Economic Sustainability) National Institute of Standards and Technology Green Building Initiative Center for Sustainable Building Research University of Minnesota Minnesota Sustainable Building Guidelines U.S. Green Building Council