18" Standard Double Wall Pipe features a "double wall" or an inner and outer pipe. The outer pipe, made of galvanized steel, provides cooling air necessary to maintain safe chimney temperatures, while the inner pipe, made of stainless steel, emits the gases out of the fireplace. Hi-Temp Woodburning Chimney. 2" clearance to combustibles is required, unless noted differently. 12" Pipe = 15" Outside Diameter. Outer Shell = Galvanized Steel. Inner Shell = Stainless Steel.