Vapor retardants limit water vapor movement in structures to prevent mold and mildew growth. They are used in walls, ceilings, floors, crawl spaces, and basements. Vapor retarders can be installed during new construction or added later, and should not be applied to both interior and exterior walls. Air barriers should be used in conjunction with vapor retardants for humidity control.
A vapor retardant is a specialized material that limits the movement of water vapor into or out of a structure. Vapor retardants are sometimes referred to as vapor barriers, although there is no material that can be used to completely stop the flow of water vapor through them. A vapor retarder is primarily used to prevent excessive amounts of moisture from condensing within the structural cavities of a building when the air reaches its dew point temperature. Condensation within these spaces can cause mold or mildew to grow and pose serious health risks to building occupants. Vapor retardants are typically used to control moisture levels in walls, ceilings, floors, crawl spaces and basements.
Vapor retardants began to be used in building construction in the 1920s. At the time, researchers believed that water vapors could only move through structural cavities in a linear fashion. In order to limit the flow of moisture into and out of a structure, builders have used the least permeable materials available to construct a vapor barrier. Further research in the 1950s indicated that air movement generally carried more moisture in and out of structures than water vapors. Based on this research, current building codes require the use of an air barrier in conjunction with a vapor retarder for adequate humidity control.
In climates with lower year-round humidity levels, a vapor barrier is usually applied to the exterior of a structure. In wetter climates, the interior walls of a building are the typical location for the vapor retarder. Most vapor retardants are installed as a flexible sheet material or as a liquid coating. Flexible materials such as laminate foils, metal foils, plastic films and treated paper are generally used as vapor retardants on the exterior of a building. Specialty primer and topcoat paints are often used as interior vapor retardants.
Vapor Retarders can be installed during new construction or added to an existing structure at any time. However, a vapor retardant should never be applied to both the interior and exterior walls of a building. Controlled amounts of moisture should always be allowed to enter and leave the structure to prevent condensation forming in the wall cavities. Air barriers such as caulking and paint stripping used in conjunction with properly placed vapor retardants are often the best defense against mold and mildew problems. Local building codes are usually the best way to determine the type of vapor retarder needed and its placement indoors or outdoors.
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