Vapor barriers control moisture movement within building assemblies. In cold climates, interior vapor barriers prevent warm interior air moisture from condensing in walls. In warm climates, exterior barriers prevent humid exterior air moisture from entering walls. Barrier material selection depends on climate and application. Polyethylene film is common but has limitations in some applications. Kraft-faced insulation provides vapor resistance. Vapor-retarder coatings applied to surfaces provide control. Installation requires continuity; seams must be sealed preventing moisture bypass. Tape designed for vapor barriers seals seams. Caulk or sealant at penetrations prevents moisture infiltration. Improper vapor barrier installation causes condensation and moisture damage. Double vapor barriers are generally avoided as they trap moisture between them. Drying capacity of walls must be ensured. If walls cannot dry, moisture accumulation causes deterioration. Materials selection must allow drying to at least one side. Timing of vapor barrier installation affects whether walls can dry. Barriers installed early prevent drying. Barriers installed after drying is complete prevent re-absorption. Climate zone and construction type determine vapor barrier placement. Building science design principles guide barrier placement for the climate.