Hempcrete is a building material made from the woody inner core of hemp stalks mixed with a lime-based binder and water. It looks and functions somewhat like lightweight concrete but serves a fundamentally different purpose: instead of holding up a building, hempcrete fills in the walls to provide insulation, soundproofing, and moisture control. It has been used in European construction for decades and was formally recognized in the 2024 International Residential Code in the United States.
What Hempcrete Is Made Of
The core ingredient is hemp hurd, sometimes called hemp shiv. This is the chopped-up woody interior of the hemp stalk, not the fibrous outer layer used for textiles. Hemp hurd is lightweight, porous, and naturally resistant to pests and mold. On its own, it’s just loose plant material. The binder is what turns it into a solid wall.
The binder is typically a blend of hydrated lime and a mineral powder like metakaolin, mixed in roughly equal parts. When combined with water and packed into a wall form, the lime undergoes a slow chemical reaction, gradually hardening around the hemp particles over weeks. A common mixing ratio is 1 part hemp hurd by weight to 1.5 parts binder by weight. In practical terms on a job site, that translates to about four buckets of hemp hurd for every one bucket of binder.
Insulation and Thermal Performance
Hempcrete’s thermal conductivity ranges from about 0.055 to 0.064 W/mK, which puts it in the category of moderate insulation. It is not as efficient per inch as rigid foam or spray foam, so hempcrete walls tend to be thicker, often 12 inches or more, to hit the same insulation targets. You can reach equivalent thermal resistance simply by increasing wall thickness, which is a straightforward tradeoff that works well in new construction but can be limiting in renovations where space is tight.
Where hempcrete differs from conventional insulation is thermal mass. Because the material is dense enough to absorb and slowly release heat, it smooths out temperature swings throughout the day. A hempcrete wall doesn’t just slow heat transfer; it stores warmth during the day and releases it at night, which can reduce peak heating and cooling loads in ways that R-value alone doesn’t capture.
Strength and Structural Limits
Standard hempcrete without any additives has a compressive strength around 29 psi. For context, conventional concrete sits in the range of 3,000 to 5,000 psi. This means hempcrete cannot support the weight of a building on its own. It is classified as an infill material, packed into a structural frame made of timber or steel that carries the actual loads.
Researchers at Penn State have demonstrated that modified hempcrete mixtures with additives can reach compressive strengths between 58 and 655 psi depending on the recipe. Those higher numbers are promising for expanding hempcrete’s role in construction, but for now, virtually all residential hempcrete projects rely on a separate load-bearing frame. The hempcrete fills the spaces between the framing, creating a continuous insulating envelope.
Fire, Mold, and Pest Resistance
Hempcrete is naturally fire-resistant. The lime binder doesn’t burn, and the hemp hurd, once encased in lime, chars slowly rather than igniting. Fire-resistance testing conducted under ASTM E 84 standards showed the material withstanding temperatures up to 575 degrees Fahrenheit. Additional testing under more rigorous standards (ASTM E 119) has been recommended to further characterize its performance in structural fire scenarios.
The high alkalinity of lime creates an environment hostile to mold growth and unappealing to insects and rodents. Unlike fiberglass or cellulose insulation, which can harbor moisture and become breeding grounds for mold, hempcrete is vapor-permeable. It absorbs and releases moisture rather than trapping it, which helps regulate indoor humidity and protects the structural frame from moisture damage over time.
Sound Absorption
Hempcrete performs well as an acoustic material. Testing shows a Noise Reduction Coefficient (NRC) between 0.43 and 0.59, depending on the mix and density. An NRC of 0.45 means the wall absorbs roughly 45% of sound energy hitting it, which is comparable to purpose-built acoustic panels. The porous structure of the hemp hurd traps sound waves rather than reflecting them, making hempcrete walls noticeably quieter than standard drywall-over-stud assemblies. For the sound reduction index, measured values range from 3 to 11 dB depending on wall thickness and composition.
Carbon Sequestration
One of hempcrete’s most distinctive properties is its carbon footprint, or rather, its lack of one. Hemp plants absorb large amounts of carbon dioxide as they grow. That carbon remains locked in the hemp hurd after harvest. The lime binder also reabsorbs CO2 from the atmosphere as it cures, a process called carbonation that continues for years after the wall is built. The net result is that one cubic meter of hempcrete sequesters roughly 300 kg of CO2 over its lifecycle. This makes hempcrete one of the few building materials that stores more carbon than is emitted during its production and transport.
By comparison, producing one cubic meter of conventional concrete releases several hundred kilograms of CO2. The difference means a hempcrete home can be carbon-negative from the walls alone, before factoring in any energy savings during occupancy.
How Hempcrete Is Installed
There are two primary installation methods, each with a distinct tradeoff between speed and flexibility.
Cast-in-place involves mixing hempcrete on-site, then packing it by hand or spraying it into temporary formwork (wooden molds) built around the structural frame. Once the hempcrete sets enough to hold its shape, the formwork is removed and the wall is left to dry. This method allows full customization of wall shapes and thicknesses but requires a significant drying period before plaster or other finishes can be applied. The drying time depends on how much water was used in the mix and the ambient conditions, but it commonly takes several weeks to months in humid climates.
Precast blocks or panels are manufactured off-site, where the hempcrete is cast into standardized shapes and allowed to dry in controlled conditions. Once cured, they’re transported and assembled on-site like masonry. The major advantage is predictability: drying is completed before the blocks arrive, so there’s no waiting on weather. The tradeoff is less flexibility in custom shapes and the added cost of manufacturing and shipping.
A third option, spray application, uses a pump to blow the hempcrete mixture onto a wall surface. This is faster than hand-packing but requires specialized equipment and experienced operators.
Building Code Status in the US
Hempcrete gained formal recognition in the 2024 International Residential Code under Appendix BL, titled “Hemp Lime Hempcrete Construction.” This is a significant milestone. Before this inclusion, builders in most US jurisdictions needed special permits or variances to use hempcrete, which added cost and uncertainty to projects. With the IRC listing, local code officials now have a standardized framework for approving hempcrete walls in residential buildings, though individual states and municipalities still need to adopt the 2024 code for it to take effect locally.
The code recognizes hempcrete as a non-load-bearing infill insulation within a structural frame, consistent with how the material has been used in practice. It was evaluated for seismic performance and found suitable, though the committee noted that further fire-resistance testing beyond the initial ASTM E 84 results would strengthen the material’s case for broader adoption.
Practical Considerations for Builders
Hempcrete walls are significantly thicker than conventional insulated walls, typically 10 to 14 inches, which reduces usable floor area. This is less of an issue in new construction where the footprint can be adjusted, but it can be a dealbreaker for retrofits. The material is also heavier than fiberglass batt insulation, though much lighter than concrete, so foundation requirements don’t change dramatically.
Cost is currently higher than conventional insulation in most US markets, driven by limited supply chains for hemp hurd and lime binders, plus the specialized labor involved. As the 2024 code adoption spreads and demand grows, prices are expected to come down. In Europe, where the supply chain is more mature, hempcrete is already cost-competitive with some conventional wall assemblies when total lifecycle costs, including energy savings and durability, are factored in.
The material’s vapor permeability is both an advantage and a design constraint. Hempcrete walls should not be sealed with impermeable coatings like vinyl wallpaper or vapor barrier paint, as this traps moisture and defeats the material’s ability to breathe. Lime-based plasters and breathable paints are the standard finishes.