Analysis Tool Targets Hidden Sources of Carbon in Mass Timber Projects
Mass timber buildings are popular for their claimed carbon neutrality and sustainability compared to conventional concrete and steel structures. But is that common assumption true across the board?
That is the question two architects at design firm Corgan have studied since a 2023 World Resources Institute report outlined sources of carbon emissions in the timber supply chain that were not being well documented. “[These] timber-harvesting practices often unaccounted for in our industry’s life cycle assessment (LCA) analysis,” says Varun Kohli, principal and director of sustainability at Corgan, New York, who partnered with Mahdi Afkhami, Corgan design researcher.
Kohli and Afkhami published their own report addressing issues impacting mass timber biogenic carbon emissions, including “slash,” the residual parts of trees including branches, twigs, roots and bark left in the forest during harvesting.
Only 35% of harvested trees make it to the building stage while the remaining 65% are often unaccounted for in calculations as they release their biogenic carbon back into the atmosphere, the report notes.
Creating a New Tool
Based on their research findings, the team designed a free carbon calculator “to help architects and designers estimate biogenic carbon emissions [in the raw material supply stage] to better manage a project’s overall embodied carbon calculations,” according to Corgan.
“Understanding the amount of CO2 equivalent released from slash in the raw material extraction phase is critical for developing more accurate and effective carbon management and mitigation strategies,” Kohli says, adding that architects and designers can use the tool to make decisions with more accurate information.
“This is a uniquely interesting tool because it highlights an aspect of the mass timber life cycle—the harvesting residuals,” says Vincent Martinez, CEO at Seattle-based climate action-oriented nonprofit Architecture 2030. “Not many tools look at residual biomass after harvest.”
After Architecture 2030 received a U.S. Forest Service Wood Innovation grant in 2023, it included the Corgan tool as one of dozens of tools and other resources for its Mass Timber Tipping Point project, Martinez says.
The tool also “adds more information and analysis about the impact of shipping mass timber from the manufacturing site to the building site on a project’s sustainability,” he says.
Mass Timber Use on the Rise
Corgan’s study comes amid global demand for wood expected to increase fourfold by 2050, the WRI report notes. “Growth in mass timber projects is skyrocketing,” says Tom Braun, mass timber consultant for the Ironworkers, AFL-CIO (IW) and the Ironworker Management Progressive Action Cooperative Trust (IMPACT). “Mass timber is here to stay.”
The Corgan study examines three slash management approaches including site compositing, pile burning and forest mastication, a fuel reduction method to reduce wildfire risk. It also considers seven tree species commonly used in mass timber projects in the U.S. including Alaska yellow cedar, Douglas fir, hemlock fir and western red cedar, which vary in level of carbon emissions from species to species.
This “allows designers to consider a lower carbon intensive timber species at every project phase, resulting in more sustainable results,” the Corgan statement notes.
Architecture 2030 research has shown that designers are interested in mass timber, but need to have a better understanding of it’s environmental implications, Martinez says. “Architects are inquisitive about materials and are doing their due diligence to better understand the supply chain.”
While LCA analysis is widely used, the industry is still developing “processes and boundaries for LCA analysis for mass timber and many other construction products and materials,” says Kohli.
“Our research and findings feel like a natural step in a maturing industry around embodied carbon,” he says. “Embodied carbon calculations as part of the design process are still fairly new for the design and construction industry.”
Many LCA Tools Available
Other commonly used tools include Whole Building Life Cycle Assessment tools such as Tally, OneClick LCA, and Athena Impact Estimator. “Tally allows users to include or exclude biogenic carbon flows in alignment with ISO standards, OneClick LCA calculates biogenic carbon storage separately without including it in the Global Warming Potential (GWP), and Athena integrates biogenic carbon flows by default across life cycle stages,” Kohli says.
UpStream, a free Excel-based tool in open beta, evaluates wood products across all life cycle stages and allows customization of end-of-life scenarios while also accounting for sourcing and forest-management-related carbon benefits (A0 impacts).
Since the Corgan tool went live in April, the researchers have been gathering feedback and further advancing sustainable design tools.
“Our research is the next step in fostering industry collaboration,” Afkhami says. “We hope this collective effort will improve carbon accounting practices and promote more sustainable construction processes.”
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