Evaluating the Carbon Footprint of Wood Buildings

Reducing greenhouse gases with high-performance structures
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That said, changing environmental conditions have made the active management of forests critical. For example, wildfire is a natural and inherent part of the forest cycle. Today, however, wildfires must be prevented from burning unchecked because of danger to human life and property. As a result, many forests have become over-mature and overly dense with excess debris, which, combined with more extreme weather, has caused an increase in both the number and severity of wildfires. The combination of older forests and changing climate is also having an impact on insects and disease, causing unprecedented outbreaks such as the mountain pine beetle—which further add to the fire risk.

Active forest management, which includes thinning overly dense forests to reduce the severity of wildfires, helps to ensure that forests store more carbon than they release. Forest management activities aimed at accelerating forest growth also have the potential to increase the amount of carbon absorbed from the atmosphere. The International Panel on Climate Change (IPCC) has stated: “In the long term, a sustainable forest management strategy aimed at maintaining or increasing forest carbon stocks, while producing an annual sustained yield of timber, fibre or energy from the forest, will generate the largest sustained mitigation benefit.”9

Whether trees are harvested and used for products or decay naturally, the cycle is ongoing, as forests regenerate and young trees once again begin absorbing carbon. But when trees are manufactured into products and used in buildings, a new phase of carbon mitigation begins.

Wood Buildings Store Carbon

Wood is comprised of about 50 percent carbon by dry weight.10 So the wood in a building is providing physical storage of carbon that would otherwise be emitted back into the atmosphere. For example, according to the Dovetail Partners report, the structure of an average U.S. single-family home stores about 9.3 metric tons of carbon, which is equivalent to 34 tons of CO2.

In a wood building, the carbon is kept out of the atmosphere for the lifetime of the structure—or longer if the wood is reclaimed and reused or manufactured into other products. Wood stores more carbon than is emitted during its harvest, production, transport, and installation—even when transported over great distances.

As part of its report, Dovetail posits that increasing the use of wood in construction could significantly enhance carbon storage in the nation's building stock. According to the Forest Climate Working Group, a coalition that collaborates on forest carbon strategy and policy recommendations, the current inventory of wood structures in the U.S. is estimated to store 1.5 billion metric tons of carbon, which is equivalent to 5.4 billion tons of CO2. Most of this resides in the nation's housing stock, about 80 percent of which is wood-frame construction. Increasing wood use to the maximum extent feasible in multi-family housing, low-rise non-residential construction, and remodeling could result in a carbon benefit equal to about 21 million metric tons of CO2 annually—the equivalent of taking 4.4 million cars off the road indefinitely.

Another study, this one published collaboratively by researchers at Yale University and the University of Washington, estimates that using wood substitutes could save 14 percent to 31 percent of global CO2 emissions and 12 percent to 19 percent of global fossil fuel consumption.11

Wood has Low Embodied Impacts

Embodied energy—which is the energy required to harvest, manufacture, transport, install, maintain, and dispose or recycle a material—also contributes to wood's light carbon footprint. Life cycle assessment (LCA) studies, which consider the environmental impacts of materials over their entire lives, consistently show that wood performs better than other materials in terms of embodied energy, air and water pollution, and greenhouse gas emissions.

One of the reasons wood performs well is that it requires far less energy to manufacture than other materials12—and very little fossil fuel energy, since most of the energy used comes from converting residual bark and sawdust to electrical and thermal energy. For example, the production of steel, cement, and glass requires temperatures of up to 3,500°F, which is achieved with large amounts of fossil fuel energy. On average, the U.S. and Canadian forest industries generate about 65 percent and 60 percent of their energy needs (respectively) from sources other than fossil fuels.

A comprehensive review of scientific literature examined research done in Europe, North America, and Australia pertaining to life cycle assessment of wood products.13 It applied life cycle assessment criteria in accordance with ISO 14040-42 and concluded, among other things, that:

▶ Fossil fuel consumption, potential contributions to the greenhouse effect, and the quantities of solid waste tend to be minor for wood products compared to competing products.

▶ Wood products that have been installed and are used in an appropriate way tend to have a favorable environmental profile compared to functionally equivalent products made out of other materials.

Increasingly, architects and engineers are utilizing LCA as an objective way to compare the environmental impacts of their material choices. This is due in part to the fact that information on LCA, including databases, tools and research, is growing. For example, the Consortium for Research on Renewable Industrial Materials (CORRIM) undertakes LCA research, concentrating on U.S. products and materials.

In 2005 and 2010, CORRIM published the results of two phases of a landmark study comparing wood-frame and steel-frame homes in Minneapolis and wood-frame and concrete homes in Atlanta (the building types most common in those parts of the country). Phase II placed an emphasis on carbon footprint, and confirmed that the carbon stored in wood products offsets many of the emissions from other products. Despite the small total mass difference resulting from substituting steel or concrete framing for wood, the Global Warming Potential (CO2 equivalent of greenhouse gas emissions, including CO2, methane, and nitrous oxide) from the steel-framed house was 26 percent greater than the house with wood walls and floors, without considering the carbon stored in the wood products. This became a 120 percent difference when the carbon stored in the wood products for the life of the house was included. Emissions from the completed, concrete wall-framed house were 31 percent greater than the wood wall house without considering the carbon stored in wood products, and 156 percent greater when these carbon stores were included in the calculation.

Evaluating the Carbon Footprint of Wood Buildings

Photo by Nic Lehoux; courtesy of Bing Thom Architects

The Athena Sustainable Materials Institute undertakes similar research and also develops and maintains LCA tools for use by North American building designers. For example, the Athena Impact Estimator for Buildings is a robust, easy-to-use software tool for evaluating the environmental footprint of whole buildings and building assemblies. Free to design and building professionals (www.athenasmi.org), it can model over 1,200 structural and envelope assembly combinations, taking into account the environmental impacts of material manufacturing, including resource extraction and recycled content, related transportation, on-site construction, maintenance and replacement effects, and demolition and disposal. It provides a cradle-to-grave life cycle inventory profile, with results covering energy and raw material flows (from and to nature) plus emissions to air, water, and land.


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Originally published in Architectural Record