VAPOR BARRIER

A vapor barrier (or more accurately, vapor retarder) is any material, typically a plastic or foil sheet, that resists passage of moisture through wall, ceiling and floor assemblies of buildings. It helps prevent interior (or exterior) moisture from penetrating into and condensing in unheated attics, basements, crawlspaces, and wall cavities when there is a significant temperature or vapor pressure differential from inside to out. This is especially important in northern climates, where there is often a great difference in temperature between the air in conditioned space and the air in unconditioned space. The vapor barrier is placed between the insulation and the conditioned space, usually stapled to the studs, but may be anywhere in the inner third of the insulated cavity (1/3 - 2/3 rule).
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