Fiberglass is used in everything from boat hulls to bathtubs to insulation. Its fire behavior is frequently misunderstood — partly because “fiberglass” can mean the glass fibers alone, or the glass fiber composite reinforced with a resin. These two things have very different fire properties.

Glass fibers themselves are not flammable — they’re made of glass, which doesn’t burn. However, fiberglass composites (like those in boats, car bodies, and tanks) use a resin matrix — typically polyester resin or epoxy resin — that is flammable. The composite as a whole is combustible because of the resin content, even though the glass fibers provide some structural support after the resin burns away.

Fiberglass Insulation

Fiberglass batt insulation (the pink fluffy rolls used in walls and attics) is made almost entirely of glass fibers. The material itself is non-combustible. However, fiberglass insulation often comes with a paper or foil facing, which is flammable. In attic or crawl space applications where the facing is exposed, this facing can contribute to fire spread. Always install insulation with the facing against the warm side and covered by drywall or another fire-rated material.

The glass fiber core of insulation provides thermal protection but doesn’t stop fire spread — fire moves through the stud cavities above or below the insulation, not through the fiberglass itself.

Fiberglass Composites in Fires

Fiberglass boat hulls, truck bodies, storage tanks, and bathtubs are fiberglass composites. In a fire, the resin matrix (polyester or epoxy) burns, releasing the glass fibers. Burning fiberglass composite produces thick black smoke and toxic fumes from the burning resin. The glass fibers themselves don’t burn but become airborne as the resin burns away, creating a respiratory hazard for firefighters and occupants.

Fiberglass composite boats are particularly challenging in marina fires — the resin burns intensely, and burning fiberglass afloat can spread fire to the water surface via flaming resin drips.

Fire Safety with Fiberglass

For building insulation: keep it covered with drywall. For fiberglass composite structures: treat them as combustible. Don’t store flammable liquids in fiberglass composite tanks unless the tank is specifically rated for those materials. For industrial fiberglass work (cutting, sanding), the dust presents a respiratory hazard but not a significant fire risk. For related reading, see our article on whether PVC pipe is flammable.