Closed cell foam is flammable. It is made from polyurethane or polyisocyanurate, both of which are petroleum-based plastics that will ignite and burn when exposed to sufficient heat. Manufacturers add flame retardant chemicals during production to slow ignition and reduce flame spread, but these additives do not make the foam fireproof. This is why building codes require protective barriers between foam insulation and occupied living spaces.
What Makes Closed Cell Foam Burn
Closed cell foam is a rigid polyurethane plastic. Like most organic polymers, it breaks down and releases flammable gases when heated. Standard polyurethane foam begins losing structural integrity at roughly 260°C (500°F), when the chemical bonds holding the foam together start to break apart. The peak rate of breakdown occurs around 335°C (635°F), at which point the material is actively releasing volatile, combustible fragments into the air.
Polyisocyanurate foam, a close relative often used in rigid insulation boards, performs noticeably better. Its chemical structure includes ring-shaped molecular groups that are harder to break apart with heat, so major breakdown doesn’t begin until about 305°C (581°F), and peak degradation is delayed to around 390°C (734°F). In cone calorimeter testing, polyisocyanurate foam produces a peak heat release of about 180 kW/m², compared to 350 kW/m² for standard polyurethane. That’s roughly 50% less heat output during the most intense phase of burning, and the peak arrives about 100 seconds later. Polyisocyanurate also leaves behind a thick char layer (about 22% of its original weight remains as char at 800°C) that acts as a protective crust, while standard polyurethane nearly vaporizes entirely, leaving only about 3.5% residue.
If your insulation is labeled “polyiso” or “PIR,” it has meaningfully better fire resistance than standard polyurethane, though it is still combustible.
Flame Retardants in Closed Cell Foam
Closed cell spray foam contains flame retardant chemicals blended into the liquid mixture before it’s sprayed. The most common one is an organophosphate compound called TCPP. In cured closed cell foam, TCPP makes up roughly 1.7% of the material by weight. Open cell foam actually contains much more, around 9.2% by weight, because its lighter, airier structure needs more chemical help to resist flames.
These flame retardants work by interfering with the chemical reactions that sustain a fire. They slow ignition and reduce the speed at which flames spread across the foam’s surface. They do not prevent the foam from burning entirely. Given enough heat and sustained exposure, flame retardant treated foam will still ignite, burn, and release toxic gases. The retardants buy time, which in a building fire can be the difference between escape and being trapped.
It’s worth knowing that TCPP and similar organophosphate flame retardants have raised health concerns. Structurally related compounds are listed under California’s Proposition 65 as carcinogenic. TCPP itself has been linked to cell toxicity and effects on fetal development in laboratory studies. These concerns are about long-term chemical exposure, not fire performance, but they’re part of the broader conversation around spray foam insulation.
Toxic Gases From Burning Foam
The fire risk from closed cell foam isn’t just about the flames. When polyurethane burns, it produces dense black smoke loaded with toxic gases. The two most dangerous are carbon monoxide and hydrogen cyanide. Carbon monoxide binds to hemoglobin in the blood, blocking oxygen transport. Hydrogen cyanide goes a step further, shutting down the ability of cells to use oxygen at all, even if it’s present in the bloodstream. Together, these gases can incapacitate and kill faster than the fire itself.
A forensic study of 38 fire victims in polyurethane foam fires confirmed the presence of both carbon monoxide and hydrogen cyanide in postmortem blood samples, along with evidence of nitrogen dioxide inhalation. Depending on the combustion temperature and ventilation conditions, burning foam can also release hydrogen halides and other irritant gases. This toxic smoke profile is a major reason building codes treat foam plastic insulation differently from materials like fiberglass or mineral wool, which don’t produce the same chemical hazards when heated.
Building Code Requirements for Fire Protection
Because of its flammability and toxic combustion products, the International Building Code requires that foam plastic insulation be separated from the interior of a building by a thermal barrier. The standard solution is half-inch gypsum wallboard (drywall), which must be installed between the foam and any occupied space. Heavy timber construction can also qualify, as can specialty materials tested to meet specific fire performance standards.
The thermal barrier serves a specific purpose: it must protect the foam from reaching ignition temperature for at least 15 minutes during a fire, giving occupants time to escape and firefighters time to respond.
In areas that aren’t occupied living spaces, such as attics and crawlspaces, the requirements are less strict. These locations typically require only an ignition barrier rather than a full thermal barrier. Ignition barriers are thinner and lighter. Fanfold panels, thin coatings, or certain specialty paints can qualify. The logic is that a fire is less likely to start in these spaces, and no one is living there, so the foam needs protection against casual ignition sources but not the full 15-minute standard.
Protective Coatings for Exposed Foam
When drywall isn’t practical, such as in attics where spray foam covers the underside of the roof deck, intumescent coatings offer an alternative. These are specialized paints applied directly over cured foam. When exposed to fire, the coating expands dramatically (some products swell up to 20 times their original thickness) to form an insulating char layer that starves the foam of oxygen and heat.
Several products are marketed specifically for this purpose, including DC315, Contego, and FIRESHELL F10E. These coatings are tested to meet the same fire performance standards as drywall when applied at the correct thickness. They’re commonly used in retrofit situations, in attics with limited headroom, and in commercial buildings where covering foam with drywall would be impractical or expensive. Not all local building departments accept intumescent coatings as a substitute for drywall, so checking with your local code authority before relying on a coating alone is a practical step.
Practical Fire Safety With Closed Cell Foam
Closed cell foam is a high-performance insulation material, but it is not inert in a fire. If you have spray foam in your home, the key factors that determine your actual risk are whether the foam is properly covered by a code-compliant barrier and whether smoke detectors are functional throughout the building. Foam that is correctly installed behind drywall in walls and ceilings poses a very different risk profile than foam left exposed in an attic or garage.
If you’re having closed cell foam installed, confirm with your contractor that the appropriate barrier (thermal or ignition, depending on location) will be installed as part of the project. If you already have exposed foam in an attic or crawlspace, an intumescent coating can bring it into compliance without a major renovation. The foam itself will always be combustible, but the system around it determines whether that matters in practice.