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When carbon accounting accuracy climbs, wood building claims sink

The transparency push scouring building specifications for carbon dioxide emission sources is striking wood products. Parties that enjoyed the cover of live tree and sawmill output narratives on carbon sequestration now face market scrutiny of net emissions from logging operations and old growth or younger forest harvesting.

A new report from the International Institute for Sustainable Development (IISD), Winnipeg, Manitoba, adds to the body of knowledge debunking wood building product proponents’ claims of low carbon bliss. “Emission Omissions: Carbon accounting gaps in the built environment” 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.

The Cement Association of Canada-funded report graphs 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 effect of carbon losses typically outside LCA—those related to soil disturbance in logging operations, forests’ variable regeneration rates, and primary to secondary forest conversion—may represent up to 70 percent of total lifecycle emissions, authors note, thus challenging the prevailing assumption wood building materials are less carbon intensive than steel or concrete.

“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,” they contend. “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 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.”

When forest regeneration rates, soil carbon loss and primary-to-new-growth forest conversion are accounted for, the cradle-to-grave embodied emissions for a wood building could be 6 percent greater than for a concrete building, according to “Emission Omissions.” While LCA is the right approach for policy-makers and building professionals aiming to decarbonize buildings, authors 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.”

IISD published the report following peer review by an eight-member team. It included 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. “Emission Omissions” builds on “Determining the Carbon Footprint of Wood,” a Portland Cement Association document we visited here in October 2016. Authors especially challenged the wood industry’s dearth of metrics abiding Intergovernmental Panel on Climate Change-designated carbon pools, concluding: “Only a small percentage of wood removed from forests ends up as durable goods or construction products.” One 2008 study [Heiken, Jelen and Stevens] they cited found “as little as 15 percent of the initial carbon stored in a live tree is retained in forest products. Most wood ends up as waste or non-durable goods.”

The green building and environmental movement was long ago saturated with data on the carbon dioxide factor in concrete, rooted in cement production. “Emission Omissions” and “Determining the Carbon Footprint of Wood,” coupled with forthcoming research and LCA modeling, will help those concerned with global GHG levels see the forests for what they do best: Harbor wildlife, create scenery and hog carbon dioxide.