Vapor retarders are used primarily in cold climates to prevent moisture present in warm indoor air (as water vapor) from entering wall assemblies and condensing on cold exterior sheathing. Where installed properly, vapor retarders such as Kraft paper or polyethylene sheeting have been used successfully for decades in conventional wall assemblies. However, changing wall construction practices which include new energy efficient materials and solutions that dramatically alter the moisture behavior of walls prompted questions from many builders on the appropriate selection of vapor retarders.
This TechNote provides an overview of building code requirements and design considerations for vapor retarders. The focus is on climate-specific recommendations for selecting interior vapor retarder products for frame walls.
Moisture can enter walls (from indoors and outdoors) as water vapor by diffusion or air leakage. It is important that both moisture migration paths are managed to reduce the risk of moisture issues. Vapor retarders – the subject of this Tech Note – are used to manage vapor diffusion. Some vapor retarder membranes help with controlling air flow as well. The reader is referred to a companion Tech Note on Air Sealing for information on practices for controlling air leakage.
A vapor management strategy should balance limiting vapor diffusion into the wall and allowing the wall to dry out should any incidental moisture accumulate in the cavity. The first is achieved by selecting an appropriate interior vapor retarder or installing a layer of exterior insulation to limit the vapor drive. The second is achieved by ensuring that at least one face of the wall is constructed with moderate-to-high permeability materials to avoid a double vapor barrier condition.