Separate vapor retarders are used in some constructions. When required,a separate vapor retarder should be installed on the warm-in-winter side of the framing. (In hot,humid climates,vapor retarders are sometimes omitted or installed outside the insulation.) Four-mil or thicker polyethylene sheeting, available in rolls,is typically placed horizontally and stapled to the face of the framing. If more than one sheet of polyethylene is required,overlap the sheets across two framing members. Foil-backed gypsum board is also an effective vapor retarder.