Builder's Best WPV401 Series - 4" x 50' Wire Helix Poly Vinyl Duct is a lightweight version of polyvinyl chloride material. It is reinforced with a steel wire helix for support, with 1-1/2" pitch for 4" diameter duct and 1" pitch for 3" and 3-1/4" diameters. This duct is useful for bathroom fan venting but is not approved for dryer venting.