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A vapor barrier is any material used for damp proofing, typically a plastic or foil sheet, that resists diffusion of moisture through the wall, floor, ceiling, or roof assemblies of buildings to prevent interstitial condensation and of packaging. Technically, many of these materials are only vapor retarders as they have varying degrees of permeability.
Materials have a moisture vapor transmission rate that is established by standard test methods. One common set of units is g/m·day or g/100in·day. Permeability can be reported in perms, a measure of the rate of transfer of water vapor through a material. American building codes have classified vapor retarders as having a water vapor permeance of 1 perm or less when tested in accordance with the ASTM E96 desiccant, or dry cup method. Vapor-retarding materials are generally categorized as: