Hemp-lime is a mineral wall. Every hemp particle is encased in a lime-and-clay matrix that gives flame nothing to spread across, nothing toxic to release, and hours of resistance before failure. Here is the technical detail behind those claims.
All EN values below are from the manufacturer’s technical data sheet; US test context is noted separately.
Fire performance is measured two ways. Reaction to fire asks whether the material itself feeds a fire: the bare block classifies B-s1,d0 — the highest band a material containing any organic content can reach — meaning minimal contribution to fire, the lowest smoke class (s1), and no flaming droplets (d0). Once plastered in lime, the finished wall surface classifies A1: non-combustible.
Fire resistance asks how long a finished wall keeps a fire on one side of it. In furnace testing to the EN standard, plastered hemp-lime walls hold integrity and insulation — flames and killing heat stay on the fire side — for three hours at 5.9″ thick, and four hours or more from 7.9″ up. A rating of EI 240 means the wall was still standing, still intact, and still cool on the far side after 240 minutes of furnace fire.
The manufacturer states it directly: the blocks do not burn, do not spread flames, and do not emit toxic gases — they withstand up to 240 minutes without burning, without emitting toxic gases, and without letting them pass through.
| Assembly | Thickness | Rating | In plain terms |
|---|---|---|---|
| Bare block | — | B-s1,d0 | minimal fire contribution, least smoke, no droplets |
| Plastered wall surface | — | A1 | non-combustible finish |
| Wall 150 | 5.9″ (150 mm) | EI 180 | 3-hour fire resistance |
| Wall 200 | 7.9″ (200 mm) | EI 240 | 4-hour fire resistance |
| Wall 250 | 9.8″ (250 mm) | EI 240 | 4-hour fire resistance |
| Envelope 285 | 11.2″ (285 mm) | ≥ EI 240 | 4-hour-plus fire resistance |
| Envelope 320 | 12.6″ (320 mm) | ≥ EI 240 | 4-hour-plus fire resistance |
Most wall assemblies fail in fire at their weakest layer — a foam board that melts, a membrane that ignites, a cavity that acts as a chimney. A hemp-lime wall has none of those layers. It is a single mass of plant fibre locked inside a mineral binder, faced with lime plaster.
The lime-and-clay matrix does the work: it is already the product of high-temperature processing, so fire finds nothing volatile in it. The encased hemp is starved of the oxygen a flame needs. And because the wall contains no petrochemical content at all, there is nothing to produce the black, toxic smoke that makes modern house fires so lethal — most fire deaths are from smoke, not flame.
This is also where the ingredient list pays off twice: the same three natural ingredients that mean nothing off-gasses into your rooms in daily life mean nothing burns off in a fire.
The EN ratings above are the manufacturer’s, from testing to the EN standards — they document how the material behaves, and every project we supply ships with the full test reports. US code recognition runs on its own track, and it is moving fast: hemp-lime construction entered the 2024 International Residential Code as Appendix BL, and fire-rated hemp-lime wall assemblies were approved by committee in October 2025 for the 2027 IRC.
US laboratories have begun confirming the same behavior. In industry testing of comparable hemp-lime assemblies, the material scored zero flame spread and zero smoke developed on ASTM E84 — the best possible result — and a full-scale hemp-lime wall passed a one-hour ASTM E119 fire test, including the hose-stream test, at an accredited US lab.
For your permit, none of this is left for you to argue: our submittal pack puts the EN fire documentation, the US precedents, and the code references in front of your engineer and building official in one file. That is the conversation we support on every project.
Full EN fire documentation, wall sections, and submittal support ship with every project enquiry.
rachel@karumaterials.com →