Section R2403 addresses foam plastic insulation including expanded polystyrene, extruded polystyrene, and polyurethane. Foam plastics have good insulating value but are combustible. All foam plastics must be protected with noncombustible barriers including gypsum board or masonry. Foam plastics in concealed spaces must meet fire-rating requirements. Attic foam plastics must be protected by dropping ceiling or board, or must be covered by fire-rated material. Crawlspace foam plastics require protection to prevent ignition from utilities.