Installation: Vapor Retarders
For most of North America, vapor retarders should be installed in exterior walls on the warm-in-winter side of the insulation (toward the interior).
For some warm and humid areas, such as Florida, the Gulf Coast and Hawaii, the vapor retarder should generally be installed facing the outside. Check local building practice and/or building codes to be sure.
Vapor retarders are not a standard recommendation for attics. Except for very cold regions and in isolated cases where there is high humidity in the house during the winter, attic vapor barriers aren’t required provided the attic is sufficiently ventilated (as a rule of thumb, one square ft. of vent opening is needed for every 150 square ft. of ceiling).
If installed in an attic, a continuous vapor retarder is usually used to reduce air infiltration. If this is accomplished and a similar air infiltration retarder is installed in sidewalls, mechanical ventilation such as a heat recovery ventilator should be installed to prevent trapping air pollutants and moisture within the house. Moisture build-up can cause mildew on the walls and ceilings.
In other warm, humid regions, especially southern coastal areas with a long cooling season and high exterior humidity, air conditioning causes continuous moisture flow from the exterior toward the interior cooled area. If a vapor retarder is used, it should be on the exterior of the wall.
In some areas of the South, it may be difficult to determine where the vapor retarder should be placed. Where there is uncertainty, it is best to follow local practice and local codes.
Never leave faced insulation exposed. The facings on kraft-and foil-faced insulation will burn and must be installed in substantial contact with an approved ceiling wall or floor construction material.
Flame-resistant foil (FSK-25) is the only insulation facing that can be left exposed.
Separate vapor retarders are used in some constructions. They should be installed to the warm-in-winter side of framing. A 2-mil nylon film (MemBrain™, the Smart Vapor Retarder), available in rolls, is rolled out horizontally and stapled to the face of the framing. It is recommended that the vapor retarder be stapled at the sides and the excess material folded back into the room. If more than one sheet of the retarder is required, a double fold should be made at the meeting of the two pieces and stapled, or the sheets may be overlapped and taped. The pieces, if stapled, should meet only at a stud or a joist.
Cover the retarder with gypsum drywall or other approved interior material, as required by local codes, as soon as the insulation and vapor retarder has been installed.
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