Hemp Insulation Fire Rating: Class A Rated Without Toxic Chemicals
Published March 29, 2026 · By Mr Hemp House®
One of the biggest misconceptions about hemp insulation: "Isn't hemp flammable?" The answer may surprise you. Hempcrete and GaiaCrete™ the hempcrete alternative achieve Class A flame spread ratings (ASTM E84) — outperforming spray foam, fiberglass, and most conventional materials — without any chemical fire retardants.
Fire Rating Comparison
| Material | Fire Rating | Toxic Smoke? | Chemical Retardants? |
|---|---|---|---|
| GaiaCrete™ Hempcrete | Class A (ASTM E84) | No | None needed |
| Spray Foam | Minutes | Yes — cyanide, CO | Required |
| Fiberglass | Melts, doesn't burn | Binder smoke | Some |
| Cellulose | Moderate | Minimal | Borate treated |
| Mineral Wool | Non-combustible | Binder smoke | None |
Why Hempcrete Resists Fire Naturally
The secret is the lime binder. When hemp shiv is mixed with hydraulic lime, the resulting material is essentially petrified plant matter encased in calcium silicate. The lime creates a mineral matrix that:
- Chars slowly rather than flashing or melting
- Produces no toxic gases — unlike spray foam which releases hydrogen cyanide
- Self-extinguishes when the flame source is removed
- Maintains structural integrity for hours during a fire
The Spray Foam Fire Problem
Spray foam is the most dangerous common insulation in a fire. When ignited, polyurethane spray foam produces:
- Hydrogen cyanide (HCN) — lethal gas
- Carbon monoxide (CO) — the #1 fire death cause
- Isocyanates — severe respiratory toxin
- Dense black smoke — zero visibility in seconds
Fire departments consistently report that spray-foam-insulated homes produce more toxic smoke and burn faster than homes with traditional materials.
ASTM & Building Code Compliance
GaiaCrete™ carries a Class A flame spread rating under ASTM E84, the standard surface burning characteristics test for building materials. No assembly-specific ASTM E119 hourly fire-resistance rating is published for GaiaCrete™ at this time.