Carbon Sequestering through Mass Timber Construction

The United States emits over 1 billion metric tons of carbon dioxide each year (1). While a major long-term goal is to reduce the amount of carbon-related energy we consume by using renewable resources and improving building and automobile efficiency, there is a growing understanding that we must also sequester the carbon that already exists in our atmosphere in order to slow the heating of the earth.

There are many natural elements on our planet that readily absorb the carbon in our atmosphere, the most significant being bodies of water, soil, and forests. Because these three elements actively pull carbon from the atmosphere and sequester it, they are referred to as “carbon sinks.”

From the manufacturing processes that create our building products to the energy used to transport and erect a structure, the construction of new buildings adds an enormous amount of carbon to our atmosphere. Modern construction uses an incredible amount of steel and concrete, which contribute to an increase in atmospheric carbon through their production and do little to remove that carbon from the atmosphere once they are set in place.

Wood construction, however, has long been a sustainable, low-carbon option for building structures as it releases very little carbon in its production and harvesting, and absorbs significant amounts of carbon post-construction.

The reduced prevalence of wood construction may be due to its lower performance in fire safety and its structural limitations when compared to steel and concrete. However, advancements in wood construction over the last few decades has addressed both of those concerns. Modern engineered mass timber has been developed to be significantly more fire resistant and able to span further in multiple directions and support higher vertical loads than traditional light frame construction.

Cross laminated timber (CLT) is a relatively new mass timber product, which gains strength through stacking kiln-dried wood boards in alternating directions and laminating them together. This product can be used in floor, wall, and roof assemblies and can be paired with other structural materials, like steel or concrete. CLT is known as a “carbon sink” product, meaning that it absorbs more carbon from the atmosphere than it releases. Growing trees sequester carbon dioxide in their fibers, making the final timber product used on construction sites “carbon negative.” This type of product promotes the use of renewable resources, carefully managed forestry, environmentally sensitive architecture, and building technology that is meant to last.

In addition to its excellent sustainability, using CLT as a major structural system also improves efficiency on the jobsite, reduces waste, and improves speed of construction. CLT is prefabricated off-site for use on built projects and can be installed quickly without specialized construction experience. Once erected, contractors are able to walk on the decking immediately - a major time savings when compared to the curing time required by concrete construction. The speed of erection allows the building to become weather-tight sooner, which may reduce construction schedules by as much as 25% (2).

As WRA's first CLT building, the Brown University Wellness Center and Residence Hall's goal was to create an environment whose aesthetic character truly supports healthy and sustainable living. The sustainable approach for this mass timber project relies heavily on the significant reduction of carbon positive building materials, the promotion of negative construction materials, and commitment to putting healthy materials on display for its daily users and visitors. Choosing to expose the CLT in this project helped to reduce the amount of man-made materials in the building such as gypsum wall board, paint, and other synthetic materials used in conventional construction.

CLT has been slow to gain traction in the US building construction market in part due to cost implications, client education, and US code restrictions. However, the environmental benefits, ease of assembly, fire resistance, and reduced construction schedule make engineered mass timber an extremely appealing building material that is likely to balloon in popularity in the next decade.

(1) EPA, Environmental Protection Agency, 14 Apr. 2022, https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gas-emissions-and-sinks.

(2) The Strong-Tie Blog Team. “What You Should Know about Cross-Laminated Timber Construction.” Building Strong, 24 Nov. 2021, https://blog.strongtie.com/what-you-should-know-about-cross-laminated-timber-construction/.