CORRIM published a 22-module research plan and protocol in 1998 to develop a life-cycle assessment (LCA) of all environmental inputs and outputs for residential structures and other uses of wood. Research was begun on the first six of those modules in 2000, targeting Pacific Northwest and Southeast supply regions of the United States; lumber, plywood, oriented strandboard (OSB), glulam, laminated veneer lumber (LVL), and I-joist wood products; and typical houses for a warm climate (Atlanta) and a cold climate (Minneapolis).
Primary data were collected from producing mills, and virtual houses were designed to code and practice and analyzed using different building materials in the framing and sheathing. Steel and wood framing were compared in Minneapolis, and concrete and wood in Atlanta. Within-wood substitution examined the use of OSB as the alternative for plywood, green lumber for dry, and I-joists for dimension lumber in floors.
The large numbers of emission and waste outputs were reduced to several environmental performance indexes including the following: air and water emissions, global warming potential, and solid waste, along with measures of energy and material resource consumption.
The substitution of steel or concrete for wood in framing involves as little as 6-10% of the mass of a house because so many components are common, such as cement foundations, windows, gypsum covering, and roofs. Even so, the change in environmental performance is much greater. Looking only at wall and floor subassemblies results in much worse percentage comparisons for concrete and steel as the amount of common materials are reduced because the roof and foundation are not considered. Substituting OSB for plywood results in a several percent increase in risk for wood framing, but because the resource is coming from lower valued sources, the base of renewable resources is significantly extended. Dry lumber increases the risk indexes over green lumber by several percent. The wood resource used in I-joists is only 65% of the wood used in dimension lumber joists offsetting the increased energy used in OSB as the major component. But the reduced material needed for I-joists increases the material efficiency for wood by 10% compared with dimension lumber floor joists. The environmental performance changes for these within-wood substitutions are all small relative to substituting steel or concrete for wood framing.
Table 2 summarizes the energy used, including the use, maintenance, and demolition phases of the life cycle. The energy used in the structure is much greater than that used for maintenance and demolition. Energy used for heating and cooling is even greater than for construction when looking over the more than 75-year life of a house. However, the present value cost of that energy is much smaller than construction, requiring a time-sensitive investment analysis to select a better tradeoff.
Carbon emissions are an important aspect when using renewable resources. Figure 2 summarizes all carbon pools that are present in the forest as it matures. It also shows that when a forest is harvested, much of the carbon is exported to product pools, with a modest increase of carbon in the combined forest and product pools over time, unlike the steady state that exists in a forest. But of greater importance, as wood products substitute for concrete or steel materials, there is a substantial avoidance of emissions by not using these fossil-fuel-intensive building materials. The combined pools of carbon in the forest, products net of processing including the bioenergy from hogfuel, and the carbon from avoiding fossil-fuel-intensive substitutes show a substantial increasing trend over time, an important consequence for carbon policy.
Average annual carbon over time-intervals
Because so much carbon is stored in the forest, forest management impacts on carbon are of considerable interest. The impact of longer rotations in the Pacific Northwest were analyzed, and although it was noted that longer rotations over time will sequester more carbon in the forest, when adding the carbon in products and the impact of product substitution, the shorter rotations stored more carbon than did the longer rotations, with the amount of carbon increasing as the time interval of interest is increased (Figure 3). In effect, any delay in producing materials, such as a longer rotation, results in the early use of more fossil-intensive products with high emission, more than offsetting any benefits of storing more carbon in the forest on long rotations. Similarly, increasing management intensity (fertilization and thinning) in the Pacific Northwest increases product output and adds another 20+% to the product and substitution carbon pools as a consequence of the increased and earlier creation of wood products. The intensively managed rotation provided 193 metric tons of carbon per hectare in all pools for a 45-year rotation looking out over an 80-year time interval compared with 164 tons for the less intensive 45-year rotation, with this difference rising to 405 tons versus 360 tons looking out over a 165-year time-interval.
The CORRIM report provides a comprehensive database that can be used for many additional studies to improve on environmental performance and contribute to the establishment of fair environmental assessment and purchasing standards.