Class ASize: 6" ID x 8" ODMaterial: Stainless steelType HT chimney pipe special augured in mineral wool insulation filling the space between stainless steel walls is only 1 inch thick, yet provides 17 times the insulating valve of brickThis optimum density of insulation stops expansion noises and resists and contains surges of heatThis allows the system to be enclosed with only 2 inch clearance to combustiblesThe "solid pack" density comprises about 85 percent airThe mixture of extremely fine fibers trap this air in the pores of the insulation and also blocks radiation heat transferTested to 2100 degrees FUL listed