A vapor retarder — usually called a vapor barrier on the jobsite — is a material that slows water vapor from diffusing through a floor, wall or ceiling, where it could condense and cause mold or rot. Materials are rated in perms (permeance). The IRC (R702.7) groups them into Class I (0.1 perm or less: polyethylene sheet, foil), Class II (over 0.1 up to 1.0 perm: kraft-faced batts) and Class III (over 1.0 up to 10 perm: latex paint on drywall). Strictly, “barrier” means Class I; most assemblies only need a retarder.
Under concrete slabs, the IRC (R506.2.3) requires a 6-mil polyethylene (or approved) vapor retarder between the slab and the base course, with joints lapped at least 6”. Many builders now use 10–15 mil sheet meeting ASTM E1745, which resists punctures from gravel and foot traffic and keeps ground moisture out of flooring adhesives.
In walls, the rule is to put the retarder on the warm-in-winter side. The IRC requires a Class I or II vapor retarder on the interior side of framed walls in climate zones 5, 6, 7, 8 and Marine 4. In hot, humid climates an interior poly sheet can trap moisture driven in from outside, so the code does not call for it there — getting the location wrong is worse than leaving it out.