The Broan-NuTone RVK1A Steel Roof Cap Kit is an easy-to-install vent for ventilation fans (bath, rec room, home gym, etc. ). It is compatible with 3" or 4" ductwork and includes a 3" to 4" adapter for convenience. This cap features both a back draft damper and bird screen, preventing outside air from entering the home and birds and small pests from entering duct work. This kit includes all materials needed for installation, including 8 ft. of 4" flexible ducting, 4" metal duct connector, 4" to 3" Reducer, nylon zip ties and mounting screws. The flexible ducting conforms to UL 181 Class 1 Air Connector and CUL S110 requirements. Not suitable for dryer venting or flat roofs.