The Broan-NuTone 636 Steel Roof Vent Cap is an easy-to-install vent for ventilation fans (bathroom, rec room, home gym, etc.). This vent cap provides reliable roof top ventilation and features both a back draft damper and bird screen, preventing outside air from entering the home and birds and small animals from entering duct work. Compatible with 3" and 4" ducting. Not suitable for dryer venting or flat roofs.