How to Calculate the Wood Carbon Footprint of a Building

Expanding the possibilities of wood building design

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Comparison of Wood Carbon Footprint

Embodied carbon of different materials can be compared if they have the same functional equivalency, which means they provide the same service for the same length of time. The difference between these two values is referred to as the substitution benefit, meaning the avoided emissions achieved by using the lower embodied carbon material instead of the higher embodied carbon material. LCA studies consistently demonstrate wood’s substitution benefits. For example, one literature review analyzed 51 studies, which provided information on 433 substitution factors. “The large majority of studies indicate that the use of wood and wood-based products are associated with lower fossil- and process-based emissions when compared to non-wood products. Overall, the 51 reviewed studies suggest an average substitution effect of 1.2 kg C/kg C, which means that for each kilogram of C in wood products that substitute non-wood products, there occurs an average emission reduction of approximately 1.2 kg C.”⁶ The study further finds that when just looking at comparisons in the construction sector there is a substitution factor of 1.3 kg C/kg C wood product, which converts to 4.76 kg CO₂ equivalent (CO₂e)/kg C in wood.

Photo: © Chad Davies

Case Study 1

Project: DPR Construction Office, 2020 WoodWorks Design Winner for Regional Excellence
Architect: SmithGroup
Structural Engineer: Buehler Engineering

Choosing mass timber for its office building, DPR Construction sought to offer its employees the benefits of biophilic design through an overbuild design. Using CLT shear walls, a first for California, this project added a second story to a 1940s-era concrete and masonry building. DPR erected this structure to achieve both net-zero energy as well as reduced embodied carbon, with mass timber acting as a carbon sink.

More on this project: www.woodworks.org/project/dpr-office

Wood Products Store Carbon

Wood is comprised of about 50 percent carbon by dry weight, and wood in a building is providing physical storage of carbon that would otherwise be emitted back into the atmosphere. In a wood building, the carbon is kept out of the atmosphere for the lifetime of the structure—or longer if the wood is reclaimed and reused or manufactured into other products. In 2013, one study estimated the global stock of carbon stored in wood products in use was approximately 19,671 Gt (billion metric tons) CO₂e, increasing an average of 315.3 Gt CO₂e/yr.⁷ The Woodworks Carbon Calculator for Wood Buildings estimates that a U.S. 2,500-square-foot building stores 40 metric tons (mt) CO₂, a 16,000-square-foot wood office building can store 150 mt CO₂, and the wood in a 50,000-square-foot school stores 380 mt CO₂e.⁸

Woodworks commissioned a study that compared a functionally equivalent big-box retail store built with wood versus steel using the Athena Impact Estimator.¹¹ It engaged Parker Structural Engineering to design comparable one-story, 54,800-square-foot buildings and had Coldstream Consulting, an LCA firm, undertake a cradle-to-grave analysis of the material effects of structure, envelope, and interior partition assemblies. The study found that the wood-framed building performed better in five out of the six impact categories, including a 38 percent reduction in global warming potential.

Whole-Building LCA (WBLCA) tools

Architects and engineers can use whole-building LCA tools to help evaluate environmental impacts of building designs. These tools use life-cycle inventory data to readily assess material choices. For example, the Athena Impact Estimator for Buildings (calculatelca.com/software/impact-estimator) gives users access to life-cycle data without requiring advanced skills. Athena does not rely on EPDs, but has built its own database. All Athena tools comply with LCA methodology standards developed by ISO 14040 and 14044 series. The Impact Estimator and EcoCalculator use data from Athena’s own datasets and from the U.S. Life Cycle Inventory Database.

Woodworks put together a table summarizing the different tools and their applicability.

Click here to view the table

It can model more than 1,200 structural and envelope assembly combinations, allowing for quick and easy comparison of design options. Results can be summarized by assembly group and life-cycle stage. Users input basic information about building geography, size, and height. A building model is developed by creating a series of assemblies, such as walls, floors, and roofs. Materials in these assemblies can be altered to determine relative impact on total building impacts. Alternatively, users can import a bill of materials from any CAD program. These materials create a life-cycle inventory and are assessed using the TRACI (Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts) methodology⁹ to create a life-cycle impact assessment, with final reporting on GHG-related impacts including global warming potential, acidification potential, human health particulate, ozone depletion potential, smog potential, and eutrophication (harmful nutrient runoff) potential.¹⁰

Other commercial license tools, such as Tally (choosetally.com) and Oneclick (www.oneclicklca.com) integrate architect and engineer software, such as Revit, to assess environmental impacts of building material lists. Tally pulls its material life-cycle inventory information from GaBi, an international life-cycle inventory database, and One-Click relies on published EPDs, which some experts warn may be not well suited for whole-building LCAs due to inconsistencies across product categories.

WBLCA tools can help architects and engineers make material design choices to reduce the environmental impacts of buildings. These can be assessed at the individual system level (e.g., flooring, wall) or entire buildings. WBLCA tools are also acceptable in many green building certification programs, including LEED and Green Globes.

Other tools may be helpful after a building has been designed. The Embodied Carbon in Construction Calculator (EC3) (www.buildingtransparency.org/en) will facilitate the comparison of product EPDs within the same material categories. It is currently in beta form, and work is being done to properly characterize wood EPDs.

The Carbon Calculator for Wood Buildings (cc.woodworks.org) focuses on the volume of structural wood in a building and estimates how much carbon is stored in the wood, the greenhouse gas emissions avoided by not using steel or concrete, and the amount of time it takes North American forests to grow that volume of wood. It does this in one of two ways:

If the volume of wood products is known (including lumber, panels, engineered wood, decking, siding, and roofing), the carbon calculator will provide a detailed estimate for that specific building. The more detailed the information, the better the results.
If volume information is unknown, users can select from a list of common building types and receive an estimate based on typical wood use.

For the more detailed calculation, users enter the nominal volume of wood in a building, and the calculator then performs necessary volume conversions, makes corrections for moisture content, and arrives at a total mass figure of wood contained in the building. The tool then uses that information to estimate the building’s carbon benefits.

No one material is the best choice for every application. There are tradeoffs associated with each, and each has benefits that could outweigh the other material choices based on a project’s design objectives. In some cases, a hybrid structural approach can be the best option.

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Originally published in September 2020

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