Toward a Decarbonized Built Environment
The building design and construction industry is at the cusp of sea change in how we quantify and consider carbon emissions in the built environment. Any strategy toward reducing the greenhouse gas (GHG) contributions of the building sector will require a rigorous, comprehensive approach to carbon emissions. Toward this end, I offer five considerations.
Project teams should become familiar with the emerging carbon intensity metrics.
The units by which we measure the equivalent carbon emissions contributions of our buildings can be confusing. The units are unfamiliar to many professionals and they are inconsistently applied. On a per unit area basis, the two most common metrics are:
kg CO2e/m2 = kilograms of carbon dioxide equivalent emitted per square meter
lbs CO2e/ft2 = pounds of carbon dioxide equivalent emitted per square foot
Conversion factors are as follows:
1 kg/m2 = 0.204816 lb/ft2
1 lb/ft2 = 4.88243 kg/m2
There is no silver bullet to fixing the problem, but existing buildings are close.
Embodied carbon matters now more than ever and as Carl Elefante, FAIA, 2018 AIA President has stated, “The greenest building is … one that is already built.”
Larry Strain, FAIA, recently put the carbon contribution of existing buildings into perspective. Currently, about six billion square feet of new construction are built every year in the U.S. By Stain's estimate, if we make reasonable assumptions about this new, code-compliant, energy-efficient component of our total building stock, it will generate the equivalent GHG emissions of about 40 million tons of CO2 every year—less than one percent of total annual U.S. emissions across all sectors.
The embodied carbon of six billion square feet of new construction will generate something on the magnitude of 350 million tons of equivalent carbon emissions—just over a third of the total annual US emissions.
However, according to Strain, the operating emissions from our existing building stock is currently around 2.3 billion tons per year—more than a third of U.S. annual emissions; a daunting contribution which presents a staggering opportunity. If we do not effectively address operating carbon from existing buildings, reduced embodied carbon in new buildings will not matter much.
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Figure 1: Annual Carbon Emission (Millions of Tons): 6 Billion Square-Feet of New Construction / 310 Billion Square-Feet of Existing Building Adapted from Larry Strain, "Time Value of Carbon" (2017) |
Anyone should gather a sense of what their project's carbon intensity is—and how it may be reduced.
The building design and construction industry is beginning to realize the relative carbon intensity of different building types. The Carbon Leadership Forum is doing some truly amazing work. The Embodied Carbon Benchmark (ECB) Study by principal investigator K. Simonen and others is giving the industry insights into the embodied carbon of various building types throughout their life cycles. The initial phase of the ECB study revealed that over 95 percent of the buildings assessed had an initial carbon intensity under 1,000 kg CO2e/m2 (or about 205 lbs CO2e/ft2) and nearly 75 percent of the office buildings assessed were less than 500 kg CO2e/m2.
Strain also outlined the relative carbon intensity of basic building types and their constituent materials/systems categories. The bottom line: foundations and structures are huge carbon contributors. Building smaller, lighter structures saves carbon emissions over larger, heavier buildings. Moreover, when a project team can renovate an existing building, the embodied carbon of foundations, structures and exterior cladding for new construction are spared—saving tremendous amount of GHG emissions from new construction.
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Figure 2: Carbon Emissions by Building Type and Material Adapted from Larry Strain, "Time Value of Carbon" (2017) |
There are limitations to our resources and tools.
There are limitations to embodied carbon data and other life cycle assessment (LCA) reporting resources and tools. Namely, the carbon intensity of on-site operations during construction and ongoing maintenance is oftentimes missing. Please be cautious of this when comparing and working with various resources and tools.
Biogenic carbon should be factored in.
Biogenic carbon is a term used to describe the carbon contained in biomass (such as wood) that is accumulated during plant growth. Researchers are making considerable progress toward quantifying biogenic carbon, but many LCA frameworks adopt a neutrality assumption regarding biogenic carbon and, therefore, do not take it into consideration. Emerging resources and tools are now leveraging biogenic carbon in order to realize a more complete inventory with regard to the life cycle carbon intensity of buildings.
