Building Construction - Moisture/Vapor Barrier
Moisture or water vapor moves into building cavities in three ways:

1. With air currents,
2. By diffusion through materials,
3. By heat transfer. Of these three, air movement accounts for more than 98% of all water vapor movement in building cavities.  A vapor retardant and an air barrier serve to reduce this problem, but are not necessarily interchangeable.

Vapor retardant slow the rate of vapor diffusion into the thermal envelope of a structure. Other wetting mechanisms, such as wind-borne rain, capillary wicking of ground moisture, air transport (infiltration), are equally important.

Extracted from Wikipedia
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