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¼ Inch Plywood (douglas fir, exterior glue) Vapor Retarders and Perm Ratings Vapor Retarder

(A perm rating is a measure of the diffusion of water vapor through a material.) The table below shows the perm rating of some common building materials that are consistent with the ASHRAE Handbook of Fundamentals and other industry sources. Types of Vapor RetardersĪny material that has a perm rating of 1 or less is considered to be an adequate vapor retarder for residential construction. In humid climates or areas where there is extensive use of air-conditioning, if a vapor retarder is required, it should be installed on the exterior side of the wall. In areas where the climate is cold in the winter, the vapor retarder should be installed inward toward the warm living space - or on the warm side in winter. Where should vapor retarder be installed? Call your local building permitting office to find out about the local building codes and practices with regard to the need for vapor retarders. It can reduce moisture movement through your walls, floors, and ceilings. The kraft facing on insulation is a vapor retarder. If enough water is present, wood rot and decay can cause significant damage. Liquid water can accumulate inside exterior walls and in roof and attic spaces. A vapor retarder helps prevent water vapor from condensing to liquid water within the structure. Building occupants, certain appliances, plants, and plumbing equipment generate moisture that is carried in the air as vapor.Ĭontrolling water vapor movement in homes is essential. Moisture Control: Utilizing Vapor RetardersĪ vapor retarder is defined as a material or system that adequately retards the transmission of water vapor under specific conditions.