Sturdy, heavy duty steel. Fingers on one side help you hook it to plenum or duct board or any other surface. Perforated with 2 wholes if you later decide to add damper to it. Galvanized that lasts Company that you can Trust METAL Ductboard Take off Start COLLAR for HVAC Duct Work. Galvanized metal for long lasting performance. Solid Metal Sheet to hold your duct well in position.