Sources: International Institute for Sustainable Development, Winnipeg, Manitoba; CP staff
A Cement Association of Canada-funded report exposes the limitations of life cycle assessment (LCA) studies whose results have thus far painted a more favorable greenhouse gas (GHG) emissions profile of wood versus concrete or steel buildings. As the main tools to measure building products’ carbon footprints at the production, use and end of life phases, LCA studies as presently executed can exaggerate the importance of embodied GHG impacts when they discount or ignore other significant life-cycle emissions, according to “Emission Omissions: Carbon accounting gaps in the built environment.”
“LCA accounting of the GHG emissions associated with different building materials can vary widely depending on how they handle various assumptions and uncertainties,” authors contend. “Biogenic carbon emissions and sequestration related to the production and end-of-life stages of wood building products hold the most significant uncertainty in existing LCAs. Whereas emissions from the production of concrete and steel are well understood, accounting for emissions and sinks in the biogenic carbon cycle of wood products is complex and requires sophisticated carbon models that can track exchanges between different carbon pools. LCA studies typically do not track biogenic carbon but simply assume that whatever carbon is harvested is replaced sustainably by new forest growth.”
Authors graph concrete and wood product cradle-to-grave embodied emissions levels under two scenarios: traditional, measuring manufacturing and construction periods; and, biogenic carbon losses related to forest management practices. The latter gauges forest regeneration rate, soil carbon loss and conversion of natural forest—criteria unique to wood. LCAs do not track or account for biogenic carbon from the extraction and end-of-life stages of wood building products, i.e. carbon losses related to soil disturbance in logging operations, variable regeneration rates of forests, and conversion of primary to secondary forests. The effect of those losses may represent up to 70 percent of total lifecycle emissions, the report notes, challenging the prevailing assumption wood building materials are less carbon intensive than steel or concrete.
“When combined factors such as forest regeneration rates, soil carbon loss and primary-to-new-growth-forest-conversion are all accounted for, the cradle-to-grave embodied emissions for a wood building could be 6 percent greater than for a concrete building,” Emissions Omission authors affirm. While LCA is the right approach for policy-makers and building professionals aiming to decarbonize buildings, they add, “Uncertainties, assumptions and omissions in LCA studies, particularly with respect to the biogenic carbon emission of wood products, suggest that comparisons across building materials are fraught with complexity. Far more transparency, consistency and rigor in LCA data and methodologies are needed to render material comparisons meaningful.
“To address embodied GHG emissions in buildings, policy-makers and building professionals need to focus equally on material efficiency and incenting decarbonization across all material manufacturing sectors,” they conclude.
“Emission Omissions: Carbon accounting gaps in the built environment” was published by the International Institute for Sustainable Development following peer-review by an eight-member team. It included Jamie Meil of Ottawa-based Athena Sustainable Materials Institute, which has underpinned LCA perspective shaping much of the Product Category Rule for Concrete and Environmental Product Declaration content for North American ready mixed and manufactured concrete.
The report is posted here.