What are Carbon Storing Building Materials?
All building products have a carbon footprint—the greenhouse gas emissions from extracting raw materials, transporting and processing them, and transporting the finished product off to the building site — also called embodied carbon. For an average building in North America, the whole building footprint is typically 300 to 400 kg CO2e/m2. (Conversations about buildings and carbon are almost always in metric, so North Americans need to be careful to convert from Imperial units when necessary) Smaller-scale, primarily wood-framed buildings average closer to 200 kg CO2e/m2.
Bio-based materials and products are made from plants that have via photosynthesis pulled carbon dioxide out of the atmosphere, released the oxygen, and used the remaining carbon to build their cells, effectively storing this carbon in the trunk, branches or stalk. When a plant dies and decays, some of the carbon is stored in the soil while the rest is released back into the atmosphere. If these plant resources are instead harvested and converted into a building product, the carbon is effectively stored for the life of the building.
Some of the stored carbon is negated by carbon emitted during harvest, process, transport, and installation. Plants with a longer cycle for renewal (trees, that is, averaging 40 years) will lose their carbon removal benefit as living resources when cut, while rapidly-renewable resources that grow and die on a shorter cycle have greater potential for carbon drawdown. Thus, the greatest opportunity for global drawdown is in faster-renewing bio-based resources such as hemp, bamboo and straw, rather than timber. Further, softwood forests only exist in limited parts of the Earth, while grains are grown almost everywhere. Straw is abundant and ubiquitous, while forests no longer are.
What are the impacts / Why is it important?
Our most common building products typically have a big carbon footprint, especially cement and steel, both because of the production energy required and for their massive widespread uses. Petroleum-based resources such as plastics and foam insulation have large footprints as well, in addition to their other toxic by-products and grievous harm to aquatic ecosystems. Widespread use of these carbon-intensive products during construction—while often used in the interest of energy efficiency—put greenhouse gases into the atmosphere now, a time when we should be significantly decreasing emissions to stave off the worst effects of climate change.
Using carbon-storing resources in place of these “big ticket” emitters, or at least choosing the lowest embodied carbon options (low-carbon concrete and cellulose insulation, for example), has the potential to turn buildings into the earth’s sixth carbon sink. This, along with reasonably efficient all-electric buildings powered with renewable energy sources, are our best path to a sustainable future.
What’s changed?
Not long ago the focus of green building was almost solely on increasing energy efficiency and using renewable energy. Both of those remain very important, especially in the energy upgrade of existing buildings, but the embodied carbon or “up front” impacts of the materials are now recognized as equally if not more important. Carbon storing construction has a primary a role to play in achieving true zero net carbon buildings.
Taking this a step further: minimally processed bio-based resources that maintain soil health through regenerative agriculture or sustainable forest management can more than double the carbon taken out of the atmosphere. Trees—which have a longer cycle to harvest—are more effective at pulling carbon from the atmosphere towards the end of this cycle, and are often best left to grow and continue absorbing carbon in favor of annual resources such as straw, the by-product of growing food. Mycelium, enzymes, bacteria and other microbial life forms are also emerging in building products as potential mass drawdown “partners”.
Costs
Beginning with insulation: Plant-based insulation often has a lower R-value, and requires additional thickness to meet an assembly’s thermal needs. For straw or hempcrete, this could make for a 10% larger footprint for the same interior space. However, the thermal mass effect and other health benefits provide a sort of happy offset to both the occupants and the planet.
Materials that can be swapped for similar systems (e.g. mass timber or mass bamboo instead of concrete or steel; cork rather than foam insulation board, etc.) shouldn’t have differing labor costs, and with scale material costs will drop as well. Unfamiliarity with these resources can result in contractors and subcontractors charging additional to factor for ‘unknowns’, but with experience will find there’s little difference. Education for the design and engineering team will follow a similar arc.
Carbon storing resources need not exact a premium in cost. The Mahonia Building, a 34,000-sf mixed-use warehouse and office building in Eugene, OR was built for $147 per square foot. The second and third floors of office space were built with typical wood framing—2×6 studs 24” on center with plywood sheathing—and infilled with BOEBS (bales on end between studs), finished with clay plaster, with some of the clay harvested from the site excavation. Clad with steel, these walls still stored 12 tons of CO2, but had they been built with more typical steel stud framing, fiberglass and foam insulation, and gypsum board, these same walls would have emitted 60 tons, and they would have cost more as well.
Where to start and things you can do right now
1. Start by understanding the embodied carbon impacts of the materials we typically build with and then focus on reducing those impacts: Use less, use better versions, and to the extent feasible utilize ultra-low carbon and carbon-storing resources instead.
2. Insulation and cladding are some the easiest places to integrate carbon-storing alternatives. For example using dense pack cellulose insulation instead of fiberglass or foam insulation products substitutes a carbon storing resource at little if any added cost. Cladding with wood or cork rather than metal or stucco is another example.
3. Utilize Embodied Carbon accounting tools to measure your progress, which will help prioritize offsetting carbon-emitting materials with some that store carbon.
4. Target an achievable goal: 75 kgCO2e/m2 represents a very good building (400 is average for larger buildings; 200 for wood-framed residential).
5. Challenge your team to explore what a carbon-storing version of a current project might look like, and then implement what strategies the project can allow.
6. Document your efforts and share them.
Links Resources and Tools
Links – Importance of Bio-Based Construction in Addressing Climate Change:
Build Beyond Zero: New Ideas for Carbon-Smart Architecture
February 2021 CLF Report
August 2021 CLF Report:
Transformative Carbon-Storing Materials: Accelerating an Ecosystem
Building with Biomass: Turning Buildings into Carbon Sinks
Financial Times – Dec. 3, 2021
Straw-inspiring: houses made of the humble bale by Paul Miles
AIA.org – May 2022
Buildings Must Become the Earth’s Sixth Carbon Sink by William Richards
The Architectural Review – June 15, 2022
The short straw: bio-based construction by Dominique Gauzin-Muller
Builders For Climate Action – 2019
White Paper #1 Low-Rise Buildings as a Climate Change Solution
Builders For Climate Action
Emissions of Materials Benchmark Assessment for Residential Construction
Natural Resources Canada – Builders For Climate Action
Yearbook 2020 Supporting the Use of Straw in Urban and Public Buildings (UK)
Bruce King, PE, is a structural engineer and author of “Build Beyond Zero” and “The New Carbon Architecture”.
David Arkin, AIA, LEED AP is a Principal of Arkin Tilt Architects, on the AIA 2030 Commitment Working Group and co-Director of CASBA (California Straw Bale Association.)
