Variable Perm Vapor Retarder

CertainTeed is advertising its MemBrain SMART Vapor Retarder, a sheet nylon vapor retarder membrane that adjusts its water vapor permeability in response to ambient humidity conditions. According to the manufacturer, under normal low-humidity conditions, the vapor retarder has a perm rating of less than 1 and functions as a traditional vapor retarder membrane. If the relative humidity within the wall system increases, for example during summer cooling or due to moisture leakage into the wall, the permeability of the membrane also increases, providing greater drying pototential and reducing the risk of moisture entrapment within the wall system itself.