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Proper Use of Vapor Retarders

Factors to consider when selecting a vapor retarder include material type, geographic location, and placement within the structure.

A vapor retarder, more commonly known as a vapor barrier, can be useful in construction when it is necessary to separate an area of high temperature and humidity from an area of lower temperature and humidity. However, the improper use of vapor barriers can cause mold and condensation issues that can result in costly damage to structures. To avoid these problems, several factors need to be taken into careful consideration including knowing when to use and not to use a vapor barrier, the type of barrier to use, and the locality of the construction and its effect on vapor barrier placement within the structure.

The material used in construction may ultimately determine the use, or non-use, of a vapor barrier. For example, natural stone or brick veneer exteriors tend to absorb and transmit moisture inward, unlike vinyl exteriors. If moisture migration is blocked by a vapor barrier, the moisture can condense on the vapor barrier and result in mold growth. In these types of walls, it is much better to allow the wall to breathe. Natural stone or brick exteriors should also have an air gap between the stone or brick to the exterior sheathing. In all types of exteriors, it is vitally important to follow the manufacturer’s recommendations.

If the decision is made to use a vapor barrier, polyethylene sheeting is a common choice; however, any building material with low vapor permeability can qualify as a vapor barrier. Materials with low vapor permeability include vinyl wallpaper and foil-backed insulation. These materials usually have a permeability of less than 0.1 perm. Six millimeter polyethylene sheeting has a permeability of approximately 0.03. By contrast, Tyvek has permeability greater than 50, meaning it is not a vapor barrier, but is vapor permeable. Tyvek and plastic sheeting should not be used interchangeably.

Lastly, the geographic location of the construction needs to be taken into careful consideration when deciding the proper placement of a vapor barrier in relation to the wall assembly. Properly placed vapor barriers should be installed on the warm, humid side of a wall. This means in Florida, the vapor barrier should be installed on the exterior of the wall assembly, and in Alaska, the vapor barrier should be installed on the inside of the wall assembly. St. Louis’ climate is invariably warm and humid in the summer, and cold and dry in the winter. This combination of weather patterns is often a source of confusion about the correct placement of a vapor barrier.

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