Thermal Barrier

Foam plastic insulation burns fast and throws off thick black smoke, so the code will not let it face a room you live in. Section R316.4 of the International Residential Code requires foam plastic to be separated from the interior of a building by an approved thermal barrier. That means not less than 1/2 inch gypsum wallboard, or a material that passes both the Temperature Transmission Fire Test and the Integrity Fire Test of NFPA 275. Put simply: from inside a finished room, you should not see or touch bare foam.

Attics and crawl spaces get an easier standard. Under Section R316.5.3, where attic access is required and the space is entered only for repairs or maintenance, a lighter ignition barrier can cover the foam instead. Listed options include 1 1/2 inch mineral fiber, 1/4 inch wood structural panels, or 3/8 inch gypsum board. Foam tested for exposed use under Section R316.6 needs no cover.

For example, a Scottsdale home with closed-cell spray foam under the roof deck ends up with a sealed, conditioned attic and no drywall on that slope. It passes because the installer either uses a product with an evaluation report for exposed use or coats the foam with an ignition barrier. The garage is stricter. Foam there sits behind 1/2 inch gypsum board, the full thermal barrier, because a garage is a common place for a fire to start.

Three layers in a house share the word barrier and do unrelated jobs. A thermal barrier is a fire rule. It buys people time to get out before the foam behind it lights. A radiant barrier is reflective attic foil that bounces the sun's radiant heat away from your ceiling. A vapor barrier is a sheet, usually polyethylene under a slab, that blocks moisture moving through an assembly. Fire, heat, water. Only the thermal barrier comes from the fire provisions, and only it is satisfied by drywall.