Smart Vapor Retarders Proper vapor transmission for all seasons Vapor retarders are intended to prevent the transmission of water vapor. Typically installed in exterior walls forming the thermal envelope, vapor retarders are required by code to be placed on the warm-in-winter side of the wall. (See IRC 2003, Section R318). Unfortunately, placement of vapor retarders based on one season may locate the vapor retarder on the wrong side of the wall during other seasons, particularly in "mixed-humid" climate such as experienced in Gulf and Atlantic States, and areas south of Kansas, Illinois, Ohio and Pennsylvania. When improperly located, vapor retarders may trap, rather than prevent, moisture accumulation in the wall cavity. Smart vapor retarders limit the transmission of water vapor from inside the house into the wall cavity. However, when humidity levels within a wall cavity are high, the vapor retarder becomes permeable, and allows drying toward the inside living space of the home. A vapor retarder can be a component of a wall, ceiling, or floor element that retards the rate at which water vapor moves under specified conditions. Products commonly used as vapor retarders include polyethylene sheets, asphalt-backed kraft paper commonly used as insulation facing, foil laminate faced insulation, and interior paints. Based upon building codes, any membrane that has a perm rating of 1.0 or less is considered a vapor retarder. While effective at retarding vapor transmission, conventional vapor retarders located within the wall section continually prevent vapor transmission, even when conditions may require more transmission, such as during the cooling season. Two different designs have been developed in Europe. One consists of a blend of nylon compounds that become more permeable as humidity increases. Another design, not yet commercial, is a spray-on, cellulose film reinforced with glass fiber, polyethylene, and latex. This latter design is intended as a one-way vapor retarder to limit transmission from one direction but allows transmission from the other direction. 
As smart vapor retarders are a new introduction into the U.S., their availability is somewhat limited. The currently available smart vapor retarder is recommended only for cold or mixed-humid climates. Hot, humid areas of the country, where outdoor relative humidity levels are continuously high, should not use MemBrain. (See Climate Map in Summary). In addition, certain interior conditions are not suitable for the use of MemBrain, including rooms of constant, high humidity such as spas or indoor pools. This vapor retarder is also not recommended for structures intentionally humidified to levels above 50% R.H. Additionally, low permeance coatings or films such as vapor-retarding paint or vinyl wallpaper should not be used with MemBrain. Smart vapor retarders shall not be used as ground covers for basements or crawlspaces. Likewise, they must not be used as vapor retarders between concrete and finish flooring materials.
Compared to conventional polyethylene vapor retarders, MemBrain is estimated to cost $0.15 to $0.20 per square foot more.
There is no operational cost associated with smart vapor retarders. There is a potential for saving, however, as proper functioning can prevent water damage to walls.
MemBrain meets BOCA, ICBO, SBCCI, and IRC model building codes.
Not Applicable
Installation of nylon film smart vapor retarders is similar to conventional 6-mil polyethylene sheet material. Care should to be taken so that joints between sheets are overlapped at least 3 inches, and that the entire perimeter of the insulated wall is covered. The smart vapor retarder is attached using staples spaced 12" to 14" apart. Fenestrations should be handled by stapling the vapor retarder to the framing at 6" to 12" spacing. Ceiling installation is performed in a similar fashion.
MemBrain comes with a one year manufacturer's warranty.
The only product currently on the market in the United States is MemBrainTM by Certainteed. Compared to conventional polyethylene vapor retarders, MemBrain is estimated to cost $0.15 to $0.20 per square foot more. The benefit derived from smart vapor retarders is for areas of the country with elevated interior wall cavity humidity levels during the cooling season, thus increasing the opportunity for drying by diffusion. | 
