Thank you for your question on the model MD6TU make up air damper. The MD6TU is designed to be used with six inch duct work and a 6" fresh air intake wall cap with only a bird screen and no damper. 641AF is Broan's aluminum 6" fresh air inlet. It is not recommended to be installed with a roof cap for air intake because the vent always needs to be clear of any obstructions such as snow. Please contact the Technical Service Group at 800-637-1453 if you'd like to discuss your installation.