A vapor retarder is a component of the building enclosure used to control moisture diffusion through the component or assembly. It is generally placed at the warm side of the enclosure assembly to control condensation within the assembly. In terms of the International Model Building Codes, there are three classes of vapor retarders used in building enclosures, categorized as follows, based on the measured water vapor transmission per ASTM E 96 dry-cup or dessicant method A:

Class II vapor retarders are generally required in colder climates – check with the local building code. Examples of Class II vapor retarders include asphalt-kraft facings, ~2 inches of medium-density SPF. Examples of Class I vapor retarders would be a 6 mil polyethylene sheet.