A Natural Choice

How Wood Contributes to Sustainability's Triple Bottom Line
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Sponsored by reThink Wood

Calculating the Impacts of Building Designs

Numerous free tools are available to evaluate the environmental impacts of building designs. The ATHENA Impact Estimator for Buildings gives users access to life cycle data without requiring advanced skills. It can model over 1,200 structural and envelope assembly combinations, allowing for quick and easy comparison of design options.* The WoodWorks Carbon Calculator focuses on carbon footprint. Users input the volume of wood in a building, and the calculator estimates how much carbon is stored in the wood, the greenhouse gas emissions avoided by not using steel or concrete, and the amount of time it takes North American forests to grow that volume of wood.**

Carbon Footprint. Because trees absorb CO2 from the atmosphere as they grow, forests are considered a means of offsetting climate change. Wood products continue to store carbon absorbed during the tree's growing cycle even after the products are in use. The amount of carbon dioxide removed from the atmosphere could be quadrupled in 100 years by harvesting regularly and using the wood instead of steel and concrete, which, though they don't produce carbon dioxide directly, have fossil fuel-intensive manufacturing processes that result in large amounts of greenhouse gas emissions.9 Trees have a two-way flow of carbon dioxide—they breathe it in during their growth cycle, storing it in their trunks, leaves and roots, until the tree rots or burns, at which point it is released back into the atmosphere. Using timber for construction locks much of the carbon into flooring, wall framing, and other wood elements, while the regenerating forest once again begins absorbing CO2.

In building materials there is always a choice—wood studs or steel studs, wood floors or concrete slab—that affects a structure's carbon footprint, says Bruce Lippke, University of Washington professor emeritus of forest resources and lead author of a paper that appeared in the June 2011 issue of Carbon Management. Using life cycle assessment, Lippke and his team estimated that replacing steel floor joists with engineered wood joists reduces the carbon footprint of the joists by almost 10 tons of carbon dioxide for every ton of wood used. Utilizing wood flooring instead of concrete slab flooring was found to reduce the carbon footprint by approximately 3.5 tons of carbon dioxide for every ton of wood used.

The paper also noted that, to maximize the carbon benefits, the best approach is to harvest before tree growth begins to taper off, and then use the wood in place of products that are most fossil fuel-intensive. However, this does not mean that all forests should be harvested this way. "While the carbon in the wood stored in forests is substantial, like any garden, forests have limited capacity to absorb carbon from the atmosphere as they age," writes Lippke. "And there's always a chance a fire will sweep through a mature forest, immediately releasing the carbon dioxide in the trees back to the atmosphere." Older forests provide immense value, though their ability to absorb carbon dioxide slows down.

Carbon Chart

A Northwest state or private forest, harvested regularly for 100 years, helps keep carbon dioxide out of the atmosphere year after year by storing carbon in long-term wood products (blue) and by substituting wood for fossil-fuel-intensive products like steel and cement, thus avoids carbon dioxide emissions during their manufacture (orange). The chart also shows carbon that remains in a sustainably managed and harvested forest (green and black); and an “emissions” line (cranberry) at the bottom, representing the energy to harvest and process wood, which is partly counterbalanced by the “mill residual” line (yellow) that represents mill wastes burned for energy in place of fossil fuels.

Chart courtesy of E Oneil/U of Washington

As the design community focuses on carbon-neutral building, the carbon benefits of individual structures are being evaluated and documented. Avalon Anaheim Stadium, a luxury apartment and retail complex in California, includes more than 2 million board feet (equivalent) of lumber and sheathing—which will continue to store nearly 4,000 metric tons of carbon for the lifetime of the building, or longer if the wood is reclaimed and used elsewhere. By using wood instead of steel or concrete, the design team also avoided an estimated 8,000 metric tons of greenhouse gas emissions, for a total potential carbon benefit of 12,000 metric tons. These savings are equivalent to the annual greenhouse gas emissions from 2,400 passenger vehicles or the operation of 1,054 homes.10

 

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Originally published in GreenSource
Originally published in November 2013

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