Some insulation is flammable or combustible, while other insulation is noncombustible. The answer depends on the material, its facing, and how it is installed.
Mineral wool and unfaced fiberglass are generally noncombustible. Cellulose is made from paper fibers but treated with fire retardants. Spray foam and rigid foam insulation are plastic based. They can burn, melt, and produce hazardous smoke when exposed to enough heat.
Insulation Fire Safety at a Glance
| Insulation type | Can it burn? | Important detail |
|---|---|---|
| Stone or mineral wool | Generally no | Common products are noncombustible and can withstand very high temperatures. |
| Unfaced fiberglass | Generally no | Unfaced fiberglass batts are classified as noncombustible under ASTM E136. |
| Faced fiberglass | The facing may burn | Kraft paper and some foil or laminated facings can be combustible even when the fiberglass is not. |
| Cellulose | It can burn or smolder | Cellulose is paper based but treated with fire retardants to reduce flame spread. |
| Spray polyurethane foam | Yes | Spray foam is an organic plastic material and normally requires a thermal or ignition barrier in building applications. |
| Rigid foam board | Yes | Polystyrene, polyurethane, and polyisocyanurate foam can ignite and produce dense smoke when burning. |
Which Insulation Is Safest Around Fire?
Mineral wool provides the strongest inherent fire performance. Stone wool is classified as noncombustible, and some products can withstand temperatures above 1,000°C without burning or melting.
Unfaced fiberglass is classified as noncombustible under ASTM E136. That does not make every fiberglass installation fireproof. A paper or laminated facing may burn, and the complete wall, ceiling, or roof assembly determines the building's fire rating.
Can Cellulose Insulation Catch Fire?
Yes. Cellulose insulation is combustible because it is made primarily from recycled paper fibers. Manufacturers treat it with fire retardants such as borates or ammonium sulfates to reduce flame spread and smoldering. Treated cellulose can meet applicable fire safety standards, but the treatment does not make it fireproof.
Cellulose needs careful installation around:
- Recessed lights
- Chimneys and furnace flues
- Water heater vents
- Space heaters
- Other heat producing equipment
U.S. Consumer Product Safety Commission requirements state that cellulose insulation must be kept at least 3 inches from the sides of recessed light fixtures and away from exhaust flues and other heat sources.
Is Spray Foam Insulation Flammable?
Yes. Spray foam insulation is combustible. Open cell and closed cell polyurethane spray foam are plastic based materials. When exposed to fire, they can ignite, melt, and release dense smoke and toxic gases, including carbon monoxide.
Building codes commonly require spray foam to be separated from occupied interiors by an approved thermal barrier, often 1/2 inch gypsum wallboard, unless a specific tested exception applies. The requirement depends on the product, location, assembly, and local building code.
Do not leave spray foam exposed in a living space, garage, attic, or crawl space unless the product and installation have been approved for that application.
What Do Insulation Fire Ratings Mean?
Terms such as Class A, flame spread index, and smoke developed index describe results from controlled laboratory tests. They do not mean that an insulation product is fireproof under every real world fire condition.
The International Residential Code generally limits insulation used in walls, floors, ceilings, attics, and crawl spaces to a flame spread index of 25 or less and a smoke developed index of 450 or less, with specific exceptions for cellulose and foam plastic.
A compliant product can still become dangerous if it is:
- Installed against a hot flue or chimney
- Placed over a nonrated recessed light
- Left exposed when a thermal barrier is required
- Installed with the wrong facing orientation
- Installed without the required clearance
- Damaged, contaminated, or incorrectly fitted
Bottom Line
Choose insulation based on its material, facing, clearance requirements, and the full building assembly. Check the product label, installation instructions, and local building code before installing or covering insulation.
If existing insulation is charred, smells strongly of smoke, surrounds an overheated light fixture, or touches a flue, do not disturb it unnecessarily. Turn off the relevant power or appliance if it is safe to do so, then contact a qualified electrician, HVAC professional, building inspector, or fire safety professional.