Definition for word: Vapor Retarder

A material which retards the transmission of water vapor. It is rated in perms ("permeance"). The smaller the perm rating, the better the water vapor resistance. A good vapor barrier should have a perm rating no greater than 1.0. NOTE: Vapor retarders are often mistakenly referred to as "vapor barriers."

Referrence from

A vapor barrier (or vapour barrier) is often used to refer to any material, typically a plastic or foil sheet, that resists diffusion of moisture through wall, ceiling and floor assemblies of buildings and of packaging. Technically, many of these materials are only vapor retarders as they have varying degrees of permeability.

Water vapor moves into building cavities by two mechanisms: diffusion through building materials and by air transport (leakage), which is usually far more significant and problematic. A vapor retarder and an air barrier serve to reduce this problem, but are not necessarily interchangeable.

Permeability, rated in perms, is a measure of the rate of transfer of water vapor through a material (1.0 US perm = 1.0 grain/square-foot·hour·inch of mercury ≈ 57 SI perm = 57 ng/s·m2·Pa). Vapor retarding materials are generally categorized as impermeable (≤1 US perm, or ≤57 SI perm), semi-permeable (1-10 US perm, or 57-570 SI perm), and permeable (>10 US perm, or >570 SI perm). There are estabilished standards for measuring the Moisture vapor transmission rate.

Vapor retarders 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

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