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The phrase vapor barrier is often used to refer to any material, typically a plastic or foil sheet, that resists passage of moisture through wall, ceiling and floor assemblies of buildings. Technically many of these materials are only vapor retarders as they have varying degrees of impermeability. This article will use vapor barrier although increasingly vapor retarder is used in building codes. vapor barriers resist moisture from penetrating through the barrier and the moisture will instead remain on the side the barrier originating the moisture.
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.
Permeability, rated in perms, is a measure of the transference of grains of water through a material at a standard vapor pressure and temperature. Vapor retarders have permeability ratings of 1.0 or lower.
Additional recommended knowledge
Materials used as vapor barriers:
Vapor Barriers have become controversial and some out of date building codes may still require their use resulting in moldy basements. Current Building Science recommendations is to limit vapor barrier/retarder usage to limited circumstances. See sources in references.
Use of vapor barriers in basement is especially not recommended.
Under concrete slabs
A slab-on-grade or basement floor should be poured over a cross-laminated polyethylene vapor barrier over 4" of permeable fill to prevent wicking of moisture from the ground (and radon)(concrete is hydrophilic).
Sources for the section on basements and crawlspaces:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Vapor_barrier". A list of authors is available in Wikipedia.|