These are excellent quality back draft dampers. They are made from heavy-gauge galvanized steel with heavy duty springs. They use both an embossed bead & rubber gasket to help keep ducting securely in place. They prevent back draft air when your fans are off.For fitting in circular ducts. Made from pre-galvanized steel with spring-loaded vanes of aluminum, the backdraft shutter can be fitted in any position and is available in the following diameters: 4″, 6″, 8″.