Thursday, December 21, 2017

Carbon Group Post 5: Wood

Perhaps you’ve noticed that big wood-framed buildings are in the news lately:

Figure 1: Composite Timber Slab at UMass Design Building

Wood is hot for good reason. Buildings account for nearly 40% of U.S. climate-altering carbon dioxide emissions, and about 20% of those emissions are related to building construction and maintenance. Materials matter, and wood structure usually has the smallest carbon footprint of any of the primary structural materials.

The just-published ASCE report, Structural Materials and Global Climate, explains why wood structure has such a low carbon footprint. When trees are converted to structural framing, including sawn lumber and engineered products such as laminated veneer lumber, glu-lam beams, and CLT, the carbon dioxide that the trees metabolized into wood fiber is sequestered. As long as that wood is protected from decay or combustion, that sequestered carbon will not contribute to climate change. In contrast, the production of other structural materials such as steel and concrete emits significant carbon dioxide emissions.

Keep in mind that the carbon balance of living forests is complex and not yet fully understood. Harvesting wood has carbon dioxide emissions impacts that are not generally considered in life-cycle assessment such as soil disturbance and burning of tree residue. Some studies show that poorly managed forests actually emit more carbon dioxide when harvested than if they had been left alone. Therefore it is good practice to specify lumber harvested from sustainably managed forests.

Here is a simple example of how you can compare the climate impact of different structural options. On a weight basis, the “embodied carbon” in cold-formed steel framing is 2.28 lbs of CO2 per lb of steel, whereas the figure for sawn lumber is only 0.15 lbs/lb. In and of itself, this information is not all that meaningful, since steel is stronger than wood; we must look at “functional equivalency.”

So, let’s say you have a 12-foot span and want to use joists at 16” o.c. The 2012 International Residential Code specifies that 2x8 SPF #2 at 16” o.c. can span 12’-3” (for the 10 psf DL, 40 psf LL case). The IRC table for cold-formed steel joists calls for 800S162-33 under the same conditions. Converting to psf of floor area, the wood framing is 1.7 psf and the steel framing is 1.1 psf. In terms of carbon dioxide emissions, then, the wood option is 0.25 psf and the steel option is 2.4 psf, nearly ten times higher (Figure 2). In practice we should look at the entire building to account for all aspects of the construction, including those that may vary between wood- and steel-framed buildings; nevertheless, this simple comparison starts a compelling argument for the climate benefits of wood construction.

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