Is insulation flammable? It depends what's in your attic

Four materials, four different answers

"Attic insulation" covers materials that have almost nothing in common chemically. Fiberglass is spun glass. Mineral wool is spun rock or slag. Cellulose is shredded paper. Spray foam and foam board are plastics. Asking whether insulation is flammable is a bit like asking whether a kitchen is: it depends what's in it, and what's near the stove.

This page takes them one at a time, using the Department of Energy's insulation guide for homeowners and the Consumer Product Safety Commission's cellulose standard.

Is fiberglass insulation flammable? The glass isn't, the extras can be

The DOE guide is precise about this: "Fiberglass itself is noncombustible, but the resin binders it contains are combustible, as are some of the facings commonly used on batts." Its material table describes kraft paper facing as flammable, and says kraft-paper facing "should not be placed in contact with a chimney, flue pipe, or recessed light fixture."

In practice, that makes unfaced loose fill the calmer version of fiberglass, and a paper-faced batt jammed against a hot flue the version to worry about. If you have faced batts in the attic, look at what the paper side is touching.

Mineral wool: the fire-resistant one

The DOE guide's table lists mineral wool, made from rock or slag "melted and spun into fibers," as non-combustible. That's the fire half of the usual mineral wool versus fiberglass debate, and it's closer than people expect: both are mineral fibers that don't burn, and the caveats the DOE attaches to fiberglass are about its binders and facings, not the glass. The same table puts its drawbacks elsewhere: it loses R-value when compressed and "may not be readily available." If sound is your reason for considering it, our soundproofing page covers that side.

Cellulose: treated paper with a federal standard

Cellulose is mostly recycled paper, and the DOE guide says it "requires chemical treatment with fire retardant." It's also regulated: cellulose insulation sold as a consumer product falls under the CPSC's Interim Safety Standard for Cellulose Insulation, 16 CFR Part 1209, which "prescribes flame resistance and corrosiveness requirements" to reduce the risk of injury "from flammable and corrosive cellulose insulation."

Two cautions from the DOE guide go with it. First: "Although it is treated with fire retardant, cellulose may smolder if temperatures become hot enough," so it "must be kept away from chimneys and metal flues." Second, wetting matters. The guide says accidental wetting and high humidity can cause cellulose to absorb water, which reduces its insulating value and can drain the fire retardant. Cellulose that has been soaked by a roof leak isn't the same material it was on install day.

For the whole comparison, fire being only one part of it, see fiberglass vs cellulose.

Spray foam and foam board: plastics need a cover

The DOE guide doesn't hedge on plastics: "Plastic foam insulation ignites at high temperatures and emits a dense smoke containing toxic gases, including carbon monoxide." The rule that follows is that plastic foam "must be covered with a thermal barrier to retard the spread of fire," and the guide gives gypsum board as an example of an adequate barrier for interior walls. Foam sprayed on the underside of a roof deck or on attic walls therefore comes with code requirements about what covers it. Our spray foam page explains where foam makes sense in a Bay Area house.

The hot spots in an attic

In most attics the fire question isn't the insulation on its own. It's the insulation next to something hot:

• Chimneys and metal flues. The DOE guide says insulation must be kept away from them. Furnace and water-heater flues that pass through the attic need their clearance kept clear.
• Recessed lights. Only fixtures rated IC can be covered. Our page on insulation over recessed lights explains the labels and the Title 24 rules.
• Knob-and-tube wiring. The DOE guide says active knob-and-tube wiring should be replaced before insulating, because insulation can cause it to overheat. Where insulation is allowed around it under the California Electrical Code's exception, Berkeley's Form 146 lists the conditions, including that "the insulation shall be noncombustible as defined in the California Building Code." Our knob-and-tube guide has the details. We don't handle knob-and-tube ourselves; we refer you to an electrician.

Wildfire: embers come in through the vents

For homes in the Berkeley and Oakland hills, the hills above San Jose, and the wildland edges of Sonoma and Napa, this is the question behind the question. And the plain answer is that the insulation isn't the weak point.

In a wind-driven fire, embers fly ahead of the flames and get in through openings. UC's fire network says vents with quarter-inch openings "have large enough gaps to allow embers to penetrate the building," that "if combustible items are located inside the vent, smoldering embers may ignite those items," and that California's Chapter 7A building standard calls for vent openings between 1/16 and 1/8 inch. A noncombustible insulation doesn't stop an ember landing on a cardboard box under the eave vent.

So if wildfire is your worry, the attic upgrade to look at first is the vents: metal mesh at 1/8 inch or finer, or vents listed for wildfire. Our ember-resistant vents page covers the retrofit, and it's easy to plan alongside new insulation.

Choosing with fire in mind

If fire is high on your list, the sources add up to a short list: clearances kept around chimneys, flues and non-IC lights whatever the material, no paper facing against anything hot, foam always covered, cellulose kept dry, noncombustible insulation wherever knob-and-tube stays in place, and ember-resistant vents if you're near wildland. If you want help deciding what fits your attic, book a free inspection or send photos.