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Vapor Barriers

A Key Component in Moisture Management Strategies

A building without an effective moisture management strategy is a recipe for trouble, as inadequate moisture control can create several devastating problems.

Moisture can cause a great deal of damage when trapped inside a wall cavity. Over time, many building materials deteriorate and fail from being repeatedly saturated. Hidden within building cavities, wet insulation loses R-value and makes the building less energy efficient. One of the most serious problems, however, is mold and mildew growth, as airborne mold spores can lead to serious respiratory ailments. By incorporating a vapor barrier and other key moisture management strategies into the building envelope, building professionals can ward off these potential moisture problems.

Usually, a thin, film-like material, such as polyethylene, vapor barriers are used to retard or prevent water vapor diffusion into a wall, ceiling or floor during the cold winter. Unfortunately, many traditional barriers can cause trouble, as they can also prevent the evacuation of moisture that may have accumulated in an insulated cavity, preventing the wall assembly from drying.

A material's water vapor transmission rate, or permeance, is the rate at which a measured amount of water vapor transfers through a known surface area under standard atmospheric pressure conditions. Water vapor permeance is defined in nanograms (ng) of water vapor, per second, per square meter, per Pascal.

Type 1 vapor barriers are very restrictive to moisture transmission, having a permeance less than or equal to 15 ng/(Pa·s·m2) of moisture passage. These barriers are required where there is cladding on the house that doesn't allow enough drying to the outside. Type 2 vapor barriers, the basic vapor barrier for ordinary conditions, are defined as having a permeance less than or equal to 60 ng/(Pa·s·m2) of moisture passage. Prevalent vapor barrier materials used over the years have been polyethylene (Type 1), asphalt-impregnated or asphalt-coated kraft paper (Type 2) and foil skrim kraft (FSK) (Type 1) – paper-backed aluminum.

Moisture management cannot depend on a vapor barrier alone. The most successful moisture management strategies combine high-performance fiberglass cavity insulation with a vapor barrier, a water-resistive barrier, an interior air barrier and an exterior wind barrier to reduce the potential for seasonal surface condensation and rainwater penetration. An innovative new product from CertainTeed Corporation simplifies such systems by acting simultaneously as a vapor barrier and air barrier – it can prevent the entrance of moisture but also allow cavity moisture to escape.

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