• 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 andalso blocks radiation heat transfer • Tested to 2100 degrees F • UL listed.