Vapor Retarder
A vapor retarder is a building material that slows water-vapor diffusion through a wall, ceiling, or floor assembly without stopping it completely. By IECC definition, vapor retarders fall in Class II (0.1 to 1.0 perm) — examples include kraft-faced batts, smart vapor membranes (Intello, MemBrain), and certain rigid foams. Modern building science usually prefers a vapor retarder over a true Class I vapor barrier because the assembly can dry inward when needed.