Air Barrier Research Paper

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Vapour diffusion retarders and air barriers are designed to keep a building healthy and strong by preventing moisture infiltration, neutralizing differences in air pressure and preventing heat exchange. Each performs a different function although some materials serve as both an air barrier and a vapour retarder.

The Difference Between Air Barriers and Vapour Retarders

Architects, engineers and others involved in building science research and construction examine the way that both air and vapour barriers protect the building shell. The engineering design for building construction must take into account factors that affect both air and moisture penetration. These factors include local climate, building orientation, style of architecture, type
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Other problems that result from air leakage include excessive indoor humidity and high energy usage. Water vapour carried by air can be up to 200 times greater than diffusion through materials.

A good air barrier has several qualities. It must be permanent and not deteriorate over time. It must be continuous and of sufficient strength to remain in place. It must stop air movement. Air barriers should be inexpensive. An air barrier should be resistant to moisture and must also be able to resist air pressure load without separating from its anchor.

Air barriers can be installed anywhere in the wall assembly—on the cool side, warm side or in the middle. However, if an air barrier will also serve as a vapour barrier, it must be installed on the appropriate side of the wall assembly. Common air barrier materials include polyethylene sheets, housewraps and well-sealed foam insulation boards. Other air barriers include gaskets used in electrical outlets and boxes, weatherstripping, caulking and polyurethane foam. Interior finishing materials such as drywall, wood panelling and sheathing are not effective air barriers if they have gaps at the joins.

Facts about Vapour
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The ability of a material to retard the diffusion of vapour is expressed as vapour permanence, or perm. Vapour retardant materials are divided into three classes of vapour permanence. Impermeable materials have a perm of 1 or less. Semi-permeable materials have a perm greater than 1 but less than 10. Vapour permeable materials have a perm of 10 or more.

There are several materials that are classified as impermeable, including polyethylene film, rubber membranes, foil and foil-faced sheathings, oil-based paints, glass, metal and vinyl wall coverings. In cold environments, impermeable materials should be placed facing the inside of the building because condensation on the inside of the wall tries to exit the structure through the wall assembly. Interior condensation may cause moisture to accumulate within the wall. In warm climates, the material should be placed on the exterior of the wall assembly to prevent moisture from entering from the outside.

Materials considered to be semi-permeable include latex paint, plywood, asphalt-coated building papers, paper and facings on fibreglass batt insulation, rigid foam insulation and polystyrene boards. Semi-permeable materials are recommended for mixed

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