Materials
Hempcrete (hemp-lime) walls
Hempcrete is hemp hurd mixed with a lime binder. In California it is a non-structural wall infill around a timber or steel frame, with clear code limits on height, weight and finish.
What the California code allows
- Hemp-lime is limited to "the nonstructural, solid infill mix of hemp hurd and its binder between or around structural and nonstructural wall framing," at 12.5–25 lb/ft³ (BL103.1).
- One storey above grade, with an average wall weight of no more than 65 psf, and at least 3 inches of hemp-lime between the face of the framing and the finish (BL103.3.1, BL103.3.4).
- Homes in Seismic Design Categories D0–D2, which cover most of California, need an engineered design by a registered design professional (BL101.1).
- Appendix BL applies only where a city or county has adopted it, or where the building official accepts it under the alternative-materials process (R104.11).
Ways to place it
Section BL103.6 allows hand-cast hempcrete in tamped lifts of 4 inches or less, spray-applied hempcrete, precast blocks laid in lime mortar in running bond, and panels. Panels need an approved design by a registered design professional (BL103.6.6).
| Density (lb/ft³) | R-value per inch | 12-inch wall (calculated) |
|---|---|---|
| 12.5 | R-2.10 | R-25.2 |
| 15 | R-1.86 | R-22.3 |
| 20 | R-1.54 | R-18.5 |
| 25 | R-1.20 | R-14.4 |
The 12-inch column is per-inch R-value × 12, before plaster. Walls with heat capacity of at least 6 Btu/ft²·°F count as mass walls under Title 24 (BL106.1).
Drying time and build speed
Cast-in-place hempcrete dries at about one week per inch of thickness from both faces, so a 12-inch wall needs about six weeks in good weather, and about twice as long against a permanent form. Plaster can start when moisture is at or below 25% at 1.5 inches deep. Precast blocks arrive dry, so plastering can begin soon after the walls go up.
Blocks you can buy
| Maker | Product and size | Published price | Structural? |
|---|---|---|---|
| Hemp and Block (USA) | Hempcrete block, nominal 4×8×16, 8×8×16, 12×8×16 in; laid in mortar | $8 / $15.50 / $21 per block | Not stated as structural; infill with framing |
| IsoHemp (Belgium) | Hemp blocks 60×30 cm face, 7.5–25 cm thick; 30 and 36 cm tongue-and-groove | No maker price; a Belgian distributor lists €32.53–107.89/m² excl. VAT | Non-structural |
| HempBLOCK International (Australia, US arm) | LB300 interlocking mortar-free block, 600×308×300 mm, 18 kg | $265 per m² of wall, indicative, currency not stated, includes estimated container shipping | Structure comes from cast elements inside the blocks |
| Just BioFiber (Canada) | Interlocking hemp-lime block with internal composite frame, 542×272×203 mm, 13 kg | No published price | Maker claims vertical load capacity; code listing not verified |
| Hempitecture (USA) | HempWool insulation, binder, hempcrete materials | No block price found | States hempcrete is not structural |
Material cost, cast in place
Hemp and Block lists binder at $45 per 50 lb bag and building-grade hurd at $50 per 33 lb bag; one bag of each yields up to 4.5 cubic feet. That is about $14–21 per cubic foot, which is about the cost per square foot of a 12-inch wall. For a 400 sq ft home with about 545 sq ft of net wall, that is about $7,700–11,500 in materials, or about $12,900 using 12-inch blocks. These are MicroHom's calculations on published unit prices, not quotes. Installed cost has no reliable published figure.
Other properties
- Moisture: water-resistive barriers and vapor retarders are not allowed on hemp-lime walls; keep hempcrete 8 inches above grade and flash the base of the wall (BL103.7).
- Metal contact: metal touching hemp-lime must be stainless or coated with epoxy, oil, bituminous paint or another approved coating; water-based paints are not allowed (BL103.4).
- Termites and mold: generally reported termite-resistant because of lime alkalinity; mold can form under coatings when wet (Muhit et al. 2024).
- Carbon: one study measured a lime-hemp concrete sample storing 307 kg CO₂ per m³ at 28 days.
Try the Block BuilderHemp suppliers
Sources
- 2025 California Residential Code, Appendix BL, Section BL101 General (gocodebook). Checked Oct 17, 2026.
- 2025 CRC Appendix BL, Section BL103 Hemp-Lime Construction (gocodebook; effective Jan 1, 2026). Checked Oct 17, 2026.
- 2025 CRC Appendix BL, Section BL105 Fire Resistance. Checked Oct 17, 2026.
- 2025 CRC Appendix BL, Section BL106 Thermal Performance. Checked Oct 17, 2026.
- 2024 International Residential Code, Appendix BL (ICC). Checked Oct 17, 2026.
- 2025 CRC Section R104, duties and powers of the building official (alternative materials, R104.11). Checked Oct 17, 2026.
- FEMA P-2192 Seismic Design Category maps for the 2024 IRC (July 2024). Checked Oct 17, 2026.
- HempStone, "The drying of hempcrete walls" (Jul 1, 2020). Checked Oct 17, 2026.
- Hemp and Block, hempcrete blocks (published prices). Checked Oct 17, 2026.
- Hemp and Block, hempcrete binder. Checked Oct 17, 2026.
- Hemp and Block, hempcrete product category (hurd). Checked Oct 17, 2026.
- IsoHemp technical data sheet, hemp blocks (2025). Checked Oct 17, 2026.
- HempBLOCK International, US pricing. Checked Oct 17, 2026.
- Just BioFiber, product specifications. Checked Oct 17, 2026.
- Hempitecture, hempcrete. Checked Oct 17, 2026.
- Muhit et al. 2024, hempcrete durability review (Teesside University). Checked Oct 17, 2026.
- "Assessment of Carbon Sequestration of Hemp Concrete" (paper on ResearchGate). Checked Oct 17, 2026.
Last updated Oct 18, 2026. Each source above was checked on the date shown.