The Master Flow 12 in. x 2-1/4 in. to 6 in. Stack Boot is designed to create a straight line between a round pipe or insulated flexible duct and a rectangular stack duct. This boot is designed with durability in mind and is made of galvanized steel. The stack boot can be installed using metal sheet screws and sealed with a building-code approved sealant or duct tape (screws, sealant and tape not included).