Data Reveals Hemp’s Low Carbon Footprint in Construction

*Jay Arehart, Assistant Teaching Professor of Building Systems Engineering at University Colorado Boulder.*

By Elizabeth Lunt

A Colorado engineering professor’s 2020 research has given builders and designers a new way to measure the way hemp-based biomaterials can store carbon in buildings. By comparing data from a life cycle assessment of hemp+lime insulation, or hempcrete, using digital tools, builders can easily see the benefits of hemp as compared to traditional construction materials.

Dynamic life cycle assessment (LCA) is a method by which building materials are tracked from production to destruction, and both carbon storage and emissions are measured. Builders for Climate Action’s BEAM Estimator, co-developed by Ottawa, Canada-based natural builder Chris Magwood, helps builders choose materials based on the amount of carbon storage versus their emissions through their lifecycle.

Jay Arehart, a teaching assistant professor in the Building Systems Engineering Department at the University of Colorado-Boulder, published a model of hempcrete carbon sequestration in his 2020 paper, On the theoretical carbon storage and carbon sequestration potential of hempcrete, in the Journal of Cleaner Production. Magwood and others have used the data from Arehart’s research to showcase hempcrete’s stellar carbon sequestration properties as a building material.

Arehart told HempBuild Mag he became curious about whether the carbon footprint of our built environment could be understood as greater than the problems of a building’s emission. As he started to study carbon storage, he realized that buildings could actually store more carbon than they emit. “I wondered,” he said, “can we dream bigger and push the limits of what we know how to do in terms of designing buildings?”

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His curiosity led him to materials that can sequester carbon, mostly so-called biomaterials that store carbon during growth. Lumber is classic in North America, but hemp, with its fast growing cycle, far outpaces wood.

“I decided to look at hempcrete because a fast-growing bio-based material with a shorter rotation period is better at storing carbon.” In combination with lime binders, which also store carbon, hempcrete becomes an attractive option for builders interested in lowering carbon emission and raising storage.

Arehart and his team studied the range of suggested hempcrete mix designs from Chris Magwood’s 2016 book Essential Hempcrete Construction because they wanted to encompass all the formulations suggested, and look at different densities for 36 binder types. The theoretical model assumes the same size hemp but mass – at a target density – is the more important measure. “Carbon is measured through the chemistry of the reactions that happen when we are creating hempcrete,” he said. In combination, the hemp and the lime binder score well in the modeling.

The result? Arehart’s calculations indicate after subtracting all of the emissions created to produce it, hempcrete stores -16kg (-7.3 lbs.) of carbon per square meter of hempcrete wall assembly.

Magwood certainly agrees on the importance of this work and pointed out that dynamic LCA counts carbon removed from the atmosphere as a positive. “The mainstream LCA world doesn’t acknowledge that taking the carbon out is important,” he told HempBuild Mag. “But we need to count it.” Arehart’s work contributes to the full-cycle measure of hempcrete’s carbon emissions and storage.

Emissions from the manufacturing of the hemp and binder are quantified at each stage: coring and crushing the limestone; growing the hemp including machinery such as tractors; production of the binder and subsequent hempcrete mix; and transporting all materials.

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All the CO2 emissions are calculated and compared against storage rates. Arehart pointed out that cementitious materials like lime and concrete mortars sequester C02 even though they also can emit. “Carbon stored in binder will stay in the binder forever even if it is chopped up. Burning is the only way it would be released” he said. .

Chris Magwood said that his team used the results from Arehart’s study as one of the sources for the BEAM estimator and that the binder number is an average of the many that Arehart studied.

Hemp is particularly good at insulating, and Arehart pointed out that a key takeaway is the importance of comparing the function that each material can provide. A kilogram of concrete is very different than one of steel. “Make the comparisons on the full life cycle of durability, strength and function,” he recommends.

*Hempcrete insulation allows builders to construct carbon-zero buildings that sequester more embodied CO2 than the other materials in the structure.*

Hemp fares very well in carbon storage comparisons. In fact, Magwood has been able to build some net-zero carbon residences with it. “When we have done the calculations on a bunch of our houses we have gotten very close to zero or actually in a few cases negative,” said Magwood. “It’s not rocket science; you have to use more materials that store than materials that emit.”

Builders are trying to balance the materials that we know have great storage capacity – like hemp – and use them in appropriate quantities as they look to use the lowest carbon emitting materials in all other areas of the building, Magwood said.

The BEAM calculator’s spreadsheet helps builders and designers select potential materials because the carbon footprint of each is listed. Based on quantity and dimensions, architects and builders can make a comparison and calculate the material choices to make a difference.

Jacob Waddell of the U.S. Hemp Building Foundation told Hempbuild Mag that now that carbon-capture is increasingly used as a decision-making tool for building materials, “the work that Jay and his group did is a foundation as we move hemp forward to be a competitive material.” The USHBF is working now on industry-wide life cycle assessment numbers that will “feed into the BEAM calculator and will be at least partially based on the work Jay has done.”