Type 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 value of brick This optimum density of insulation stops expansion noises and resists and contains surges of heat This allows the system to be enclosed with only 2 inch clearance to combustibles The solid pack density comprises about 85 percent air The mixture of extremely fine fibers trap this air in the pores of the insulation and also blocks radiation heat transfer Tested to 2100 degrees F UL listed.