This 3 inch (7.6cm) flexible duct tubing is UL listed and is constructed of heavy mil black PVC. It is intended for use in conjunction with the Wall Duct Boxes - the 4x8 and 4x4 boxes that are for use with the HiFlo and HiFlo Lite Fan Units. This tubing will connect to the exhaust end of the rough-in box and carry the heated air inside the walls or ceiling to vent to another area in the home.