Yes, rigid foam insulation is flammable. All common types, including expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate (polyiso), are organic materials that ignite readily and spread flame quickly when exposed to fire. This is why building codes require rigid foam to be covered with a protective barrier in nearly all installations.
Why Rigid Foam Burns
Rigid foam insulation owes its excellent thermal performance to its chemical makeup: polymers like polystyrene and polyurethane filled with tiny pockets of trapped air or gas. These organic compounds are inherently combustible. EPS, for example, is about 98% air by volume, which makes it a superb insulator but also means fire can move through it rapidly. Modified polyurethane foams can achieve thermal conductivity as low as 0.022 W per meter-kelvin, outperforming many other insulation types, but that performance comes with a tradeoff in fire safety.
Manufacturers add flame retardant chemicals during production to slow ignition and reduce flame spread. For decades, the primary additive in polystyrene foams was a chemical called HBCD, which was effective but persistent in the environment. It has since been classified as a persistent organic pollutant under international treaty, and the industry has largely transitioned to newer alternatives. These additives don’t make the foam fireproof. They buy time by raising the temperature needed for ignition and slowing how fast flames travel across the surface, but the foam will still burn if fire exposure continues.
What Happens When Rigid Foam Catches Fire
Burning rigid foam is dangerous not just because of the flames but because of what it releases into the air. Polystyrene foams produce thick black smoke filled with toxic compounds. Polyurethane-based foams (including polyiso) generate carbon monoxide and hydrogen cyanide during incomplete combustion. Hydrogen cyanide is particularly lethal. A forensic study analyzing postmortem blood samples from 38 victims of polyurethane foam fires confirmed that inhalation of these gases, not burns, was the primary cause of death.
This means that in a house fire, burning rigid foam insulation can make conditions unsurvivable faster than many other building materials. The smoke is dense, visibility drops quickly, and the toxic gas concentration rises within minutes. This is the core reason building codes treat rigid foam differently from mineral-based insulation like fiberglass or rock wool, which are noncombustible.
How Building Codes Address the Risk
The International Building Code requires foam plastic insulation to be separated from the interior of a building by an approved thermal barrier. The most common solution is half-inch gypsum wallboard (standard drywall). This barrier serves two purposes: it prevents a small fire from reaching the foam in the first place, and if fire does reach it, the gypsum board delays ignition long enough for occupants to escape.
There are limited exceptions. Some products have passed specific fire tests that allow them to be installed without a full thermal barrier, typically in attics, crawl spaces, or exterior sheathing applications where the foam is on the cold side of the wall assembly. But as a general rule, if you can see rigid foam insulation on an interior wall or ceiling, it either needs to be covered or it’s a code violation.
Differences Between Foam Types
Not all rigid foams behave identically in a fire, though all are combustible.
- EPS (expanded polystyrene): The white beadboard you see in coolers and packaging. It ignites at relatively low temperatures, melts and drips as it burns, and can spread fire to surrounding materials through those flaming droplets. It shrinks away from flame quickly, which can expose framing or other combustibles behind it.
- XPS (extruded polystyrene): The colored boards (pink, blue, green depending on brand). Similar fire behavior to EPS but denser and slightly slower to ignite. It also melts and drips.
- Polyisocyanurate (polyiso): Typically faced with foil or fiberglass mat. It chars rather than melts, which gives it somewhat better fire performance than polystyrene foams. The facing material also provides a degree of protection. However, it still burns and produces highly toxic smoke including hydrogen cyanide.
Polyiso’s charring behavior is one reason it’s often preferred in commercial roofing and wall assemblies where fire ratings matter. But “better than polystyrene” is not the same as fire-resistant, and it still requires code-compliant barriers when used in occupied spaces.
Practical Safety Considerations
If you’re installing rigid foam in a garage, basement, or anywhere inside your home’s living envelope, plan to cover it with drywall. This is not optional under most local building codes, and it’s the single most important step you can take to reduce risk. Rigid foam left exposed in a workshop, for instance, means a stray spark from a grinder or a small electrical fault could escalate into a toxic fire within seconds.
For exterior applications like foundation insulation or continuous wall sheathing, the foam is typically protected by soil, siding, or stucco on one side and the wall assembly on the other. These installations generally don’t require an interior thermal barrier, but the foam still needs to meet flame spread and smoke development ratings. Most commercial rigid foam products carry a Class 1 or Class A flame spread rating when tested with their flame retardant additives intact, meaning they meet the minimum standard for building materials. That rating reflects controlled lab conditions, not what happens in a real fire after prolonged exposure.
Storing rigid foam boards on a job site also carries risk. Stacking unprotected foam near heat sources, welding operations, or in direct sun where temperatures climb can create hazardous conditions. Treat it like what it is: a combustible material that needs to be kept away from ignition sources until it’s properly installed and covered.