Hemp insulation comes in two main forms: fiber batts that fit between wall studs like fiberglass, and hempcrete, a lime-and-hemp mixture you pack into wall cavities. Manufacturing fiber batts requires industrial machinery most people don’t have access to, but mixing and installing hempcrete is a realistic DIY project. Here’s what goes into both processes and how to approach them.
How Hemp Fiber Batts Are Made
Commercial hemp batts go through a multi-stage industrial process that starts in the field. After harvest, hemp stalks undergo retting, a controlled decomposition (usually by leaving stalks on the ground exposed to moisture) that loosens the outer fibers from the woody core. Once the fiber bundles turn white and separate easily, they’re ready for the next step.
Decortication uses heavy machinery to mechanically strip the fibers from the stalk. The separated fibers then pass through rollers that soften them and increase flexibility. A combing process shortens and untangles the fibers into smooth, parallel strands roughly two feet long, while removing leftover wood particles. Finally, carding machines clean, separate, and align the fibers into a uniform web.
To turn that web into insulation batts, manufacturers typically use one of two methods. Needle-punching pushes barbed needles repeatedly through layered fiber mats, mechanically locking them together. Thermobonding blends a small percentage of polyester or bio-based plastic fibers into the hemp, then heats the mat so those fibers melt slightly and act as glue. The finished batts deliver an R-value of about 3.7 per inch, which is comparable to fiberglass and mineral wool batts.
This isn’t something you can replicate at home. If you want hemp batt insulation, you’re buying it. Expect to pay roughly $3 per square foot for R-13 batts sized for standard 2×4 stud walls. That’s more expensive than fiberglass (typically under $1 per square foot) but competitive with other natural insulation options.
Making Hempcrete: The DIY Option
Hempcrete is where home builders and renovators can get hands-on. It’s a mixture of hemp hurd (the chopped woody core of the stalk), a lime-based binder, and water. The result is a lightweight, breathable wall fill that insulates, regulates moisture, and resists mold. It’s not structural, so it always needs a load-bearing frame, but it works well packed between timber studs or into temporary formwork.
The Endeavour Centre, a well-known natural building school, uses a binder made from 50% hydrated lime and 50% metakaolin (aiteiteiteite processed clay product). Their mix ratio is 1 part hemp hurd to 1.5 parts binder by weight. Translated into volume, that’s 4 buckets of hemp hurd to 1 bucket of binder (half lime, half metakaolin) in a standard mortar mixer.
Water is the trickiest variable. You mist it into the mixer gradually rather than pouring it in. The goal is a mix that’s just moist enough to hold its shape when you pack it like a snowball in your hands, but fragile enough that you can break it apart with a gentle squeeze. Too much water weakens the final product and dramatically slows drying time.
Mixing and Placing Hempcrete Step by Step
Start by setting up temporary formwork on both sides of your wall frame. Plywood panels or OSB screwed to the studs work well. Leave one side open so you can pack the mix in, then move the formwork up as you go.
Add the dry binder to your mixer first, then the hemp hurd. Let them combine for a minute or two before misting in water. Mix until the hurd is evenly coated and passes the snowball test. Pack the hempcrete into the formwork in layers roughly 6 to 8 inches deep, tamping each layer lightly. You want it compacted enough to eliminate voids but not so compressed that you lose the air pockets that give hempcrete its insulating properties.
After packing, remove the formwork within a day or two so the walls can begin drying. Hempcrete needs significant drying time, often 6 to 8 weeks depending on climate, wall thickness, and airflow. Good ventilation speeds this up considerably. The walls will feel cool and damp until they’ve fully cured.
Thermal Performance and R-Value
Hempcrete has a lower R-value per inch than hemp batts. Lab testing puts its thermal conductivity between 0.067 and 0.134 W/(m·K), depending on density, which translates to roughly R-2 to R-2.5 per inch. A typical 12-inch hempcrete wall delivers around R-24 to R-30, which is adequate for many climates but may fall short in extremely cold regions without supplemental insulation.
What the R-value alone doesn’t capture is hempcrete’s thermal mass. The dense lime-and-hurd matrix absorbs heat during the day and releases it slowly at night, smoothing out temperature swings in a way that lightweight batt insulation doesn’t. In moderate climates, this effect can make hempcrete walls feel more comfortable than their R-value suggests.
Fire Resistance
Raw hemp fiber is combustible, but the treatments used in both batts and hempcrete significantly improve fire performance. Commercial hemp batts are typically treated with boric acid, which promotes char formation on the fiber surface. When exposed to flame, boric-acid-treated hemp loses only about 30% of its weight over 10 minutes of burning, compared to 74% or more for untreated fibers. The boric acid melts and forms a protective layer that limits oxygen reaching the fiber.
Hempcrete has a natural advantage here because the lime binder is mineral and non-combustible. The result is a material that chars on the surface but doesn’t sustain a flame or release toxic gases the way synthetic foams can.
Moisture and Mold Resistance
One of hempcrete’s strongest selling points is its ability to manage moisture without trapping it. Hemp-lime composites are highly vapor-permeable, with permeability ratings measured between 3.6 and 10.1 in laboratory testing. That means moisture passes through the wall rather than condensing inside it, which is the opposite of how a conventional vapor-barrier wall assembly works.
This breathability is paired with a naturally hostile environment for mold. The lime binder creates a highly alkaline surface, with a pH between 12.4 and 13.8 in conventional mixes. For reference, bleach has a pH around 12.5. Very few microorganisms survive at that level of alkalinity, which is why hempcrete walls rarely develop mold problems even in humid climates. This same alkalinity, combined with the mineral content, makes hempcrete unappealing to insects and rodents.
Building Code Status
Hemp insulation products like batts are installed the same way as any batt insulation and don’t face special code barriers. Hempcrete has had a more complicated path, but that changed in 2024 when the International Code Council added Appendix BL to the International Residential Code, specifically covering hemp-lime construction. This appendix allows hempcrete in buildings up to two stories in low-seismic-risk areas without requiring custom engineering.
There’s an important catch: the IRC is a model code, and its appendices are optional. Each state or local jurisdiction has to formally adopt Appendix BL before it becomes enforceable. The IRC serves as the residential code basis in 48 states (all except Wisconsin and Arkansas), so adoption is possible nearly everywhere, but you need to check with your local building department before starting a hempcrete project. Some jurisdictions may require you to submit engineering documentation even if the appendix hasn’t been adopted locally.
Sourcing Materials
Hemp hurd is available from agricultural suppliers and specialty building material retailers, often sold in compressed bales. Make sure you’re buying hurd (the woody core), not hemp fiber, as they serve different purposes. Hydrated lime is widely available at masonry supply stores. Metakaolin is less common but can be ordered from specialty cement suppliers.
If you’d rather skip the mixing and just want insulation you can install like fiberglass, hemp batts from brands like HempWool are sold through building material distributors. They come sized for standard stud cavities and install with friction fit, no stapling required. Unlike fiberglass, hemp fibers don’t irritate skin or lungs during handling, which makes the installation process noticeably more pleasant.