Hempcrete, Hemp Boards and Carbon: What the Studies Actually Show
Every few months somebody tells me hemp is going to save the planet one wall at a time. Maybe. But a claim like that has to survive a building inspector, a lender and a life-cycle spreadsheet before it means anything to a Kansas grower. So I went through the code book, the peer-reviewed studies and the manufacturers' own carbon paperwork for hempcrete, hemp insulation batts and hemp wood. The short version: the carbon story is real, it's conditional, and the fine print matters more than the brochure.
Hempcrete, and what the code says
Hempcrete is chopped hemp hurd, the woody core of the stalk, mixed with a lime-based binder and packed around a frame. The 2024 International Residential Code now has a whole appendix for it, Appendix BL, which defines hemp-lime as a nonstructural, biocomposite insulation infill [1]. Nonstructural is the key word. Your studs or posts carry the house. The hempcrete insulates, adds thermal mass and gives the plaster something to grab.
Before anybody starts mixing, read the first line. The appendix says its provisions are not mandatory unless specifically referenced in the adopting ordinance [1]. In plain English, it's in the model code, but it only applies where your city or county adopted it. Call the building department first. A few more things the appendix spells out [1]:
- Density range: 12.5 to 25 lb/ft³ (200 to 400 kg/m³).
- Insulation value drops as density rises: R-2.10 per inch at 12.5 lb/ft³, R-1.86 at 15, R-1.54 at 20 and R-1.20 at 25.
- Fire: hemp-lime walls do not have a fire-resistance rating. A rating has to come from testing under ASTM E119 or UL 263.
- Seismic: some higher seismic design categories require an engineered design by a registered design professional.
Think about that R-value table for a second. Light and fluffy insulates better. Dense and heavy insulates worse. As you'll see in a minute, the dense mixes are also the ones that can blow the carbon math. Same lever, two prices.
The carbon math, both directions
Hempcrete stores carbon two ways. The hemp pulled CO2 out of the air while it grew, and the lime binder slowly re-absorbs CO2 as it cures, a process called carbonation. The most careful look I found is a 2020 University of Colorado Boulder study that modeled 36 hempcrete formulations in a 1 m² wall built to the same insulating value [2].
- Carbonation won back 18.5% to 38.4% of the emissions from making the binder [2].
- Counting the hemp's stored carbon, total life-cycle emissions can go negative, down to −16.0 kg CO2e per square meter of wall [2].
- But some mixes ended up as net emitters, up to +8.16 kg CO2e, especially high-density mixes (over 300 kg/m³) made with portland cement [2].
- The same paper notes that earlier studies put hempcrete walls anywhere from −1.6 to −79 kg CO2e/m², depending on the functional unit, assumed lifetime, method and whether stored carbon was counted [2].
A UK life-cycle study of a 300 mm timber-framed hemp-lime wall landed in the same neighborhood: 82.7 kg of CO2 sequestered per square meter over 100 years, and a net life-cycle reduction of 36.08 kg CO2e [3]. A range from −1.6 to −79 isn't a rounding error. It's a reminder that the binder, the density and the bookkeeping decide the answer. Hemp doesn't get a free pass because it's hemp.
Hemp boards, batts and hemp wood
Hempcrete gets the headlines, but the fiber and the stalk go into factory products too. Here is what the makers publish, and what their own paperwork says.
- Insulation batts (Hempitecture HempWool). 92% industrial hemp fiber and 8% low-melting polymer binder, with fiber sourced half from Montana and half from Idaho, made in Jerome, Idaho [5]. A cradle-to-gate life-cycle assessment by the University of Idaho's Integrated Design Lab reports −0.251 kg CO2 per m² at a thermal resistance of RSI-1, counting stored biogenic carbon [5]. Caveat Cradle-to-gate stops at the factory door. It doesn't count what happens to the batt at the end of its life.
- Hemp wood flooring (HempWood, Murray, Ky.). Crushed hemp stalks and a soy-based, no-added-formaldehyde adhesive, pressed under 3,000 tons of pressure to roughly 60 lb/ft³, with a Janka hardness of 2,200 lbf. The company calls it carbon-negative [6].
- What HempWood's own EPD shows. The company's Environmental Product Declaration (issued Sept. 1, 2022, using 2021 data) reports total global warming potential, including biogenic carbon, of +1.84 kg CO2e per m² over 75 years and +0.45 kg for a single installation, both in the no-vacuum maintenance scenario [7]. Note Small numbers, but positive, so "carbon-negative" isn't what that declaration shows. It also covers the 2021 HempWood Natural Flooring product, which may differ from what's sold today.
- Hemp-fiber insulation panels, peer-reviewed. An Italian life-cycle study counted the CO2 a crop absorbs: one hectare yielding 15 tons of hemp came out at about −26.01 t CO2eq, with about −5.52 t allocated to the technical fiber by mass [4]. It modeled an 85% hemp, 15% polyester panel against rockwool. Abstract only The abstract doesn't state the head-to-head result, so I'm not quoting one.
What it means for a Kansas grower
Here's the part nobody puts on the brochure. In 2025, the whole country harvested 21,693 acres of fiber hemp, producing 67.3 million pounds worth $13.5 million, down 13% from 2024 [8]. That's a rounding error next to corn. A building-materials market can't grow faster than the fiber and hurd supply behind it, and that supply needs decorticators, buyers and code officials who've read Appendix BL.
- Hurd is the hempcrete input. If you're growing fiber, the woody core you separate out is what goes in the wall. A processor nearby is the whole ballgame.
- Ask about the binder before you brag about carbon. Lime-based, lighter mixes are the ones that came out carbon-negative in the modeling. Dense, cement-heavy mixes can flip positive [2].
- Read the EPD, not the tagline. Third-party declarations exist for some hemp products. Use them, and hold everybody, us included, to what they actually say.
- Check local code adoption. Appendix BL only applies where it's been adopted [1].
My take: hemp building materials are one of the best honest reasons to grow fiber hemp in the Midwest, because the carbon benefit is measurable and the code path finally exists. But I won't sell anyone a miracle. I can't control how fast the market grows. I can control whether what I tell you holds up when somebody checks.
Key takeaways
- Hempcrete is in the 2024 IRC as Appendix BL, as a nonstructural insulation infill, and only where locally adopted [1].
- Most modeled hempcrete mixes store more carbon than they emit, down to −16.0 kg CO2e/m², but dense cement mixes can reach +8.16 [2].
- A UK hemp-lime wall study found 82.7 kg CO2 sequestered per m² over 100 years, a 36.08 kg net reduction [3].
- Manufacturer numbers vary: HempWool's cradle-to-gate LCA is slightly negative [5], while HempWood's 2022 EPD totals are slightly positive [7].
- Supply is the bottleneck: 21,693 harvested acres of US fiber hemp in 2025 [8].
Sources
- International Code Council, 2024 International Residential Code, Appendix BL: Hemp-Lime (Hempcrete) Construction: https://codes.iccsafe.org/content/IRC2024P1/appendix-bl-hemp-lime-hempcrete-construction
- Arehart JH, Nelson WS, Srubar WV III, “On the theoretical carbon storage and carbon sequestration potential of hempcrete,” Journal of Cleaner Production 266: 121846 (2020). Full text (author manuscript, NSF PAR): https://doi.org/10.1016/j.jclepro.2020.121846 · https://par.nsf.gov/servlets/purl/10174020
- Ip K, Miller A, “Life cycle greenhouse gas emissions of hemp–lime wall constructions in the UK,” Resources, Conservation and Recycling 69: 1–9 (2012). Abstract only: https://doi.org/10.1016/j.resconrec.2012.09.001 · https://research.brighton.ac.uk/en/publications/life-cycle-greenhouse-gas-emissions-of-hemplime-wall-construction/
- Zampori L, Dotelli G, Vernelli V, “Life cycle assessment of hemp cultivation and use of hemp-based thermal insulator materials in buildings,” Environmental Science & Technology 47(13): 7413–7420 (2013). Abstract only (Politecnico di Milano repository): https://doi.org/10.1021/es401326a · https://re.public.polimi.it/handle/11311/970216
- Hempitecture, HempWool® Life Cycle Assessment, Integrated Design Lab, University of Idaho (manufacturer-published, cradle-to-gate): https://www.hempitecture.com/wp-content/uploads/2024/08/Hempitecture-HempWool-LCA.pdf
- HempWood (Fibonacci LLC), Frequently Asked Questions (manufacturer claims): https://hempwood.com/pages/frequently-asked-questions
- Fibonacci LLC, Environmental Product Declaration: HempWood® Natural Flooring, ASTM International Declaration No. 360, issued Sept. 1, 2022: https://pcr-epd.s3.us-east-2.amazonaws.com/828.HempWood%20Flooring_EPD.pdf
- USDA National Agricultural Statistics Service, National Hemp Report, released April 16, 2026 (PDF): https://www.nass.usda.gov/Publications/Todays_Reports/reports/hempan26.pdf
All links checked Oct. 18, 2026. “Abstract only” means we read the publisher or repository abstract, not the full paper. Manufacturer figures are the companies’ own published numbers. Carbon results depend on mix design, system boundary and end-of-life assumptions. This is not engineering, code or investment advice; check with your local building department.
