Our duct flanges are constructed of steel for durability, as well as powder coated to withstand rust that can form in humid environments. Offered in standard sizes including 4", 6", and 8". Duct flanges are meant for use with Air Duct, Light Proof Ducting, Insulated Ducting, and Terra Duct (as well as other standard sized ducting). These flanges have predrilled holes for attaching to sheet rock as well as plywood walls and also include an additional steel ring and bolt set for connecting to sheet metal duct work. They also have rounded edges to make them easy and safe to work with and install.