Ignition barrier
Also called: thermal barrier, intumescent coating, fire barrier over spray foam, IRC R316
A code-required covering over spray foam in attics and crawl spaces entered only for service: an intumescent coating ($0.50–$1.00 per sq ft) or a listed material like 1/4-inch plywood. Foam in occupied or garage spaces needs the stronger thermal barrier — usually 1/2-inch drywall.
Spray foam is a plastic, and it burns. The residential code (IRC R316) requires it to be separated from the living space by a thermal barrier — 1/2-inch gypsum drywall or an equivalent tested assembly — anywhere people occupy, including garages. In attics and crawl spaces that are entered only to service equipment, a lighter ignition barrier is enough: 1/4-inch plywood, 3/8-inch particleboard, 1/4-inch hardboard, corrosion-resistant steel, or a spray-applied intumescent coating that swells into a char layer in a fire. The coating is the usual choice on a roof deck: $0.50–$1.00 per square foot, so $750–$2,000 on a typical attic.
Exemptions that matter: foam on a rim joist up to 3.25 inches thick needs no barrier, and some foam products are tested and listed for use without an ignition barrier in attics and crawl spaces at a specified thickness — the contractor should be able to show the listing.
It’s the line most often missing from a spray foam quote and from DIY kit math. An inspector will ask for it, and so will a buyer’s home inspector when you sell.