There are three purposes for duct insulation: to retard the flow of heat, to reduce the amount of duct-transmitted sound, and/or to act as a vapor barrier. To retard the flow of heat, ducts may be insulated on the inside with duct liner, or on the outside with duct wrap. To reduce the amount of duct-transmitted sound, a product called sound liner may be used. As a vapor barrier, ducts are usually wrapped with a vapor sealable material such as foil or plastic.