A vapor retarder is a material with permeance ratings between 0.1 and 10 perms that slows but does not completely stop moisture vapor diffusion through building assemblies. Vapor retarders balance moisture control with the ability to allow assemblies to dry, making them more suitable than full barriers for many applications. Common materials include kraft paper, latex paint, and class II or III foam insulation.
Permeance Classifications
Class I retarders (0.1 perms or less) nearly stop moisture movement. Class II retarders (0.1 to 1.0 perms) include kraft paper and some paints. Class III retarders (1.0 to 10 perms) include latex paint and certain housewraps. Climate zone and wall assembly design determine appropriate retarder class, with cold climates typically requiring Class I or II protection.
Advantages Over Vapor Barriers
Vapor retarders allow controlled drying in both directions, reducing moisture accumulation and mold risk compared to rigid barriers. They provide adequate moisture control while allowing assemblies to recover from construction moisture or water intrusion. Building science research increasingly favors retarders over complete barriers for improved durability and indoor air quality in modern construction.