Vapor barrier

A vapor barrier (or vapour barrier) is any material used for damp proofing, typically a plastic or foil sheet, that resists diffusion of moisture through wall, ceiling and floor 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/m²·day or g/100in²·day. Permeability can be reported in perms, 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). 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:

Vapor diffusion retarders are normally available as coatings or membranes. The membranes are technically flexible and thin materials, but sometime includes thicker sheet materials named as "structural" vapor diffusion retarders. The vapor diffusion retarders varies from all kinds of materials and keep updating every day, some of them nowadays even combined the functions of other building materials.

Materials used as vapor retarders:

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 retarder and an air barrier serve to reduce this problem, but are not necessarily interchangeable.

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.

The industry has recognized that in many circumstances it may be impractical to design and build building assemblies which never get wet. Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior. So, the use of vapor barrier should be taken into consideration. Their use has already been legislated within the building code of some countries (such as the U.S., Canada, Ireland, England, Scotland & Wales). How, where, and whether a vapor barrier (vapor diffusion retarder) should be used depends on the climate. Typically, the number of heating degree days (HDD) in an area is used to help make these determinations. A heating degree day is a unit that measures how often outdoor daily dry-bulb temperatures fall below an assumed base, normally 18°C (65°F). For building in most parts of North America, where winter heating conditions predominate, vapor barrier are placed toward the interior, heated side of insulation in the assembly. In humid regions where warm-weather cooling predominates within buildings, the vapor barrier should be located toward the exterior side of insulation. In relatively mild or balanced climates, or where assemblies are designed to minimize condensation conditions, a vapor barrier may not be necessary at all.

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