A new evolution of hemp and lime, Hemp-lime (Hempcrete) is now in the IRC (International Residential Code). This accomplishment was a decades long project of the US Hemp Building Foundation (USHBF). The USHBF explains, “Hempcrete insulation has been used in Europe for about 30 years, but couldn’t be included in US building codes until industrial hemp was legalized in the United States with the passage of the 2018 Farm Bill.”
If you’re about to ask, “What happens when a hemp-lime home burns?”, meaning will you get high, the simple answer is “No” and “No”. No, industrial hemp does not contain any significant amounts of tetrahydrocannabinol (THC) – the molecule that causes a cannabis high when heated. And, No, hemp-lime will not burn.* In fact, ASTM test results in early 2020 scored a whopping “0,” the best possible rating on a scale of 0 to 450 for flammability. (ASTM methods and scope) Of course that is only for the hemp-lime itself. Consideration of other exposed flammable elements such as wood framing and trim must be considered in the design process.
*Fire department hemp-lime fun at 3000 degrees: https://youtu.be/K_tNTXi4RdA?si=I1w_fS8T4D2k9cs7
What is Hemp-Lime?
Hemp-lime is a biocomposite material, consisting hemp hurds (a.k.a. hemp shives) – the woody inner particle of hemp stem – mixed with lime-based binder and a small amount of water. Sometimes called “hempcrete”, this name is a bit of a misnomer because the name does not indicate if Portland cement, lime, or a combination is used as a binder. It does not indicate what kind of wall system it is.
Lime from limestone can be processed in many ways. Lime binders can include different additives. These variations in additives will produce a range of density and insulation values with different thermal properties, and various crystal lattice structural properties. The lighter the mix, the more energy efficient. Thicker walls add thermal mass, strength and insulation.
Keep connected here for future articles about different properties of lime for building, and specifically for hemp-lime.
A Quick Look at Hemp
Hemp yields more plant mass per year than a 50-year-old forest, actively absorbing carbon dioxide. During the growing season, one hectare of hemp can absorb up to ten tons of CO2 from the atmosphere. The woody inner hurd of the hemp stalk are obtained during first processing. The outer fiber is separated and used for textiles, rope, paper and other fiber-based products.
IRC Applications
The hemp-lime that has been approved in the IRC as a 2024 addendum is currently for nonbearing building material and wall infill systems. It can be built per the code for single and double family residential designs up to 2 stories in milder seismic design categories. An approved engineered design is required for multi-family dwellings, or in the more extreme seismic design categories.
Benefits of Hemp-lime (Hempcrete):
• Natural, non-toxic walls
• Highly mold, insect, pest and fire resistant
• Very low embodied energy & waste
• High R-value insulation
• Relatively low impact to the earth to obtain the materials
• Works for floor insulation & ceilings
• Moderates interior humidity beautifully
• Alkalizes to deter mites & bugs
• Significant EMF reduction
• Can be formed for domes and curves
• Sequesters carbon – carbon negative
• Super sustainable and regenerative
Keep Learning About Hemp-lime
Future articles will focus on the following topics:
- Hemp in AEC
- A deep dive into lime for construction
- Hemp-lime construction per the IRC
- Inspiring Hemp-lime designs and completed builds
- Hemp-lime resources
Please leave a comment below if you have topic suggestions or specific questions.
Photo credits: Casey House, Hempire USA, Lucid 9 Design