Masonry Magazine July 2007 Page. 42
Moisture Management
There are several issues that need to be noted regarding the application of air barriers:
Some air barrier materials also can act as weather-resistive barriers. In some cases, they can act as weather-resistive barriers and vapor retarders, as well as their primary function. As such, it is imperative that the wall designer evaluates the proper location of the air barrier within the wall assembly and specifies the correct type of air barrier for the application.
Air barriers can only be effective if they are installed in a continuous manner with all penetrations and seams sealed. Unsealed penetrations through air barriers can render them ineffective.
Most vinyl wall coverings act as a vapor retarder. In warm humid climates, application of a vapor retarder on the inner surface of the wall can result in condensation to form on the cool side of the wall (the interior). This typically results in mold growth on the gypsum sheathing.
If an air barrier also is used as a weather-resistive barrier (such as a building wrap), all penetrations through it have to be sealed and properly flashed to resist water penetration.
Air barriers will have to be designed and installed to resist wind loads. While they are concealed within the wall cavity, wind loads can transfer to the air barrier causing it to separate from its substrate. This is particularly a problem with mechanically-attached air barriers.
Vapor Retarders
WITH THE EXCEPTION of metals, all building materials are permeable to water vapor (ie., they allow movement of water vapor molecules through them when subject to differential water vapor pressure). Some materials, such as polyethylene, are less permeable than others, such as CMU or gypsum. Likewise, certain coatings are less permeable than other coatings.
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