Superior Snap Lock 24" Section Double Wall Pipe for 12" Standard Wood Burning ChimneyProducts wood burning pipe features a "double wall" or an inner and outer pipe. The outer pipe (15 3 8") provides cooling air necessary to maintain safe chimney temperatures, while the inner pipe (12"), made of stainless steel, emits the gases out of the fireplace. What's Included Double Wall Pipe Features Available in different lengths Double Wall Pipe Snap Lock For use with Superior 12" Standard Wood Burning Chimney (12 3 8" Inner Diam. 15" Outer