Quantifying sustainability: How we measure green

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    As sustainable construction principles move from niche markets to mainstream construction, significant questions emerge: What attributes make a building "green"? Where is the line drawn? Is simply being "less bad" enough to be called "green"? Or must a building go farther, and actually improve the environment? Presently, a building’s level of sustainability is likely measured by one of the available rating systems. While each system has unique attributes, they share similarities in that they award points based on achieving defined goals in areas of site selection, water and energy efficiency, material use, and indoor environmental quality. However, the ratings generally do not provide quantitative comparisons of the relative environmental impacts of competing alternatives. Life cycle assessment (LCA), a technique that evaluates a product’s environmental impact, is beginning to influence the rating systems, and it is likely to play a larger role in coming years.

    The United States Green Building Council’s (USGBC) LEED rating system, (www.usgbc.org) was introduced in 1998. The LEED system was the first widely promulgated tool to establish clearly defined sustainability goals and award levels of certification based on third-party verification. While initially only applicable to new construction, a number of variations have since launched to address renovations and building operations, as well as subsets of new construction. The USGBC also developed application guides that provide advice on the application of LEED to special project types, such as schools and retail. The LEED system has gained great popularity and is the most frequently used rating system for U.S. commercial construction. While LEED does not currently include LCA explicitly, future versions of LEED will likely incorporate provisions for, or be more clearly informed by, LCA.

    Green Globes, an environmental assessment tool applicable to commercial buildings, was developed by The Green Building Initiative (www.thegbi.org). It is based on a similar Canadian rating system, and was introduced in the United States in 2004. Green Globes uses a web-based interface where users complete a series of questionnaires at various project stages. Following completion of the applicable questionnaire, a report discussing the environmental benefits of each design strategy is automatically generated. If a rating is desired, an additional fee is required for a Green Globes trained and registered professional to provide third-party verification. Green Globes contains features of interest to the structural engineer, some of which are not included in LEED. It awards credits for using an integrated design process as well as for designing to provide durability, adaptability, and disassembly. Green Globes also awards materials credits if the selection of the structural system is informed by LCA.

    The Green Guide for Healthcare (www.gghc.org), while neither a sustainability rating system nor a LEED product, is a sustainable design toolkit addressing issues unique to healthcare facilities. The Green Guide is self-certifying and does not contain threshold levels. The forthcoming LEED for Healthcare rating system uses the Green Guide as its starting point, and it is therefore likely that many of the credits will be similar. As other LEED products evolve, they may also include relevant credits from this system. The Green Guide contains integrated design, design for flexibility, and design for durability credits. Furthermore, it does not permit fly ash from power plants co-fired with hazardous or medical waste or tire-derived fuel to contribute to the recycled materials credits.

    As mentioned previously, LCA uses inventory analyses from all stages of a product’s life cycle to perform quantitative comparisons of multiple environmental impacts. Because of the large amount of product data analyzed while performing an LCA, tools are available to facilitate the process.

    Development of these tools, however, involves many assumptions, and it is essential that the user understand the implicit assumptions when interpreting results. The Athena Institute (www.athenasmi.ca), a Canadian-based non-profit, and the National Institute of Standards and Technology (NIST) both developed LCA tools. Athena offers two tools for analyzing buildings: The Impact Estimator, a software tool that models the environmental impacts of building systems; and The EcoCalculator, commissioned for use in Green Globes as a free, simplified spreadsheet-based program that summarizes the environmental impacts of building assemblies.

    NIST’s tool, Building for Environmental and Economic Sustainability (BEES), can be downloaded for free at www.bfrl.nist.gov/oae/software/bees. BEES performs a life cycle analysis on a variety of construction assemblies and allows the user to perform quantitative assessments of environmental and economic impacts for competing alternatives. For example, BEES can compare the life cycle effects of using a 100-percent portland cement concrete to alternatives with various fly ash or slag replacement percentages. BEES contrasts multiple alternatives simultaneously and allows the user to drill down to investigate how each life cycle stage contributes to each environmental impact.

    As structural engineers, we may not need to know every detail of each rating system, but it is important that we understand how each system evaluates the materials we specify. While current efforts to better populate LCA inventories do not necessarily mean that we will be regularly performing life cycle analyses, we will likely be exposed to a more frequent use of LCA data to substantiate sustainability claims, and we must understand the LCA process to make informed decisions.

    Dirk M. Kestner, P.E., LEED AP, is a staff engineer at Simpson Gumpertz & Heger Inc., and chairman of the ASCE Structural Engineering Institute’s Sustainability Committee. He can be reached at dkestner@sgh.com. The committee website is www.seinstitute.org/committees/sustainable.cfm.