Vapor retarders or vapor diffusion retarders are materials that restrict the penetration of moisture through the ceilings, walls, floors, basements, crawl spaces or slab-on-grade foundations of the building. Vapor retarders are supplied in the form of specially treated paper, thin plastic sheeting or low-permeance paint. They are usually referred to as vapor barriers, but this is misleading since they do not completely stop the penetration of vapor but merely restrict or retard it.
Materials with a perm rating of less than 1.0 are considered as vapor retarders. Perms or permeability is a unit of measurement used to gauge the ability of a material to retard the diffusion of water vapor.
Types of Vapor Retarders
Membranes
Membranes are generally thin and flexible materials that resist water diffusion. They typically include materials such as rigid foam insulation, reinforced plastics, aluminum and stainless steel. They are usually mechanically fastened and sealed at the joints.
Coatings
Any paint or coatings effective at restricting most water vapor diffusion is considered as a vapor retarder.
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