Hempcrete Calculator: Complete Design Guide
What is Hempcrete?
Hempcrete (hemp-lime composite) is a bio-based construction material made from hemp shiv (the woody core of the cannabis sativa plant) bound with Natural Hydraulic Lime (NHL). It has been used in building construction for over 30 years, particularly in France and the UK.
Unlike conventional masonry, hempcrete is non-structural — it serves as an insulating, vapour-permeable infill within a structural frame (timber or steel). Its combination of low thermal conductivity (λ = 0.06–0.12 W/m·K), high thermal mass, and hygrothermal buffering makes it unique among insulation materials.
Mix Design & Binder Selection
The standard hempcrete mix ratio by mass is 4:1:1 (hemp shiv : NHL binder : water). NHL 3.5 is the most commonly specified binder, providing an initial set of ~14 days and full carbonation over 12 months. NHL 5 is preferred in cooler or wetter climates for its faster set.
Thermal & Hygrothermal Properties
Hempcrete thermal conductivity: λ = 0.06 + (density − 200) / 500 × 0.06 W/m·K. A 300mm hempcrete wall at density 450 kg/m³ achieves U ≈ 0.28 W/m²K — often meeting energy codes without additional insulation. Its moisture buffering value (MBV ~2.5 g/m²·%RH) passively regulates indoor humidity.
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Carbon Sequestration
Hemp sequesters approximately 1.2 kg CO₂ per kg of dry shiv. A 1 m³ hempcrete wall (density 450 kg/m³) contains ~203 kg hemp, sequestering ~243 kg CO₂ permanently in the wall. Even after NHL binder production emissions (~0.4 kg CO₂/kg), hempcrete is net carbon negative — a significant advantage over conventional masonry.
FAQs
Is hempcrete structurally strong?
No — hempcrete is non-structural and cannot support building loads. It must be used as infill within a timber or steel structural frame. Its compressive strength (~0.2–1.0 MPa) is far lower than conventional masonry (10–30 MPa).
How long does hempcrete take to dry?
A 300mm hempcrete wall takes approximately 4–8 months to reach safe moisture content for plastering in a temperate climate (65% RH, 18–20°C). High humidity or low temperature significantly extends drying time. Always verify with a moisture meter before applying render.
Can hempcrete be used in wet climates?
Yes, with proper detailing. Hempcrete must be protected from driving rain by render or cladding and from ground moisture by a raised floor slab. Properly detailed hempcrete performs excellently in wet temperate climates, as demonstrated by decades of use in the UK and France.