Hempcrete Calculator

Hempcrete is a bio-composite material made from hemp shiv (the woody core of the hemp plant) mixed with a lime-based binder and water. It is used as breathable insulation infill within a timber frame. It is not load-bearing but provides good insulation, hygroscopic moisture buffering and biogenic carbon storage.

The Tradical "Thermal Walling" / UK Hempcrete wall mix is roughly 90 kg hemp shiv, 200 kg lime binder and 180 litres of water per cubic metre of wall — a ratio of about 1 : 2.2 : 2 by weight, cast to a placed density near 330 kg/m³. In bag terms that is about 2 × 22 kg binder sacks per 20 kg shiv bale. Note the mix is binder-heavy: there is more lime binder than shiv by weight, the opposite of the old "2:1 hemp-to-binder" rule of thumb. Lighter roof and floor mixes use less binder per m³. Always confirm the exact proportions against your supplier’s current data sheet.

A practical rule of thumb is about one week of drying per 25 mm (one inch) of thickness, per exposed drying face, in moderate, well-ventilated conditions. A 300 mm wall cast between temporary shutters and left open on both faces is therefore dry enough for finishes in roughly 6 weeks (~42 days); cast against a permanent form so only one face dries, it takes about twice as long (~12 weeks). Cold, damp or poorly ventilated conditions extend this — do not apply impermeable finishes until the wall has dried out.

Wall-density hempcrete has a thermal conductivity (λ) of roughly 0.06–0.09 W/mK. Calculated to BS EN ISO 6946 (including the Rsi 0.13 and Rse 0.04 surface resistances) a 300 mm wall gives about 0.22–0.28 W/m²K — it does NOT meet the Approved Document L 2021 Table 4.2 limit of 0.18 W/m²K for a new wall in an extension or existing dwelling. Around 350–400 mm (≈0.17 W/m²K at 350 mm) is needed to comply. Renders add a little further resistance.

The hemp shiv stores biogenic carbon — peer-reviewed studies use about 1.84 kgCO₂ per kg of dry shiv, so a typical wall locks up on the order of 100–170 kgCO₂ per cubic metre. The lime binder, by contrast, emits CO₂ during manufacture and only re-absorbs part of it slowly through carbonation, so this calculator counts only the shiv storage and does not credit binder carbonation. Whether a finished wall is net carbon negative depends on the binder type and life-cycle boundary used in the assessment.