A vapor barrier is a material with a permeance rating of 0.1 perms or less that blocks moisture movement through building assemblies. Vapor barriers prevent water vapor from diffusing into wall cavities, attics, and crawl spaces where it can condense and cause mold, rot, and structural damage. Polyethylene sheeting, foil-faced insulation, and rubberized membranes serve as effective vapor barriers.
Performance Ratings and Materials
Vapor barrier effectiveness is measured in perms (permeability rating). A true vapor barrier has less than 0.1 perm rating. Polyethylene film (0.0 perms), foil-faced insulation (0.0 to 0.1 perms), and rigid foam boards (0.0 to 2.0 perms) provide varying degrees of vapor resistance. Installation location depends on climate zone, with barriers typically placed on the warm side of insulation.
Installation and Effectiveness
Proper vapor barrier installation includes continuous sealing at seams, edges, and penetrations. Incorrect placement traps moisture in wall cavities, worsening condensation problems. Modern construction often favors vapor retarders over complete barriers, allowing buildings to dry more effectively while still controlling moisture.