Flexible Aluminum Ducting, 4" X 25', Constructed of a special aluminum alloy that prevents corrosion and metal fatigue, is U.L. and C.U.L. 181 class 0 air duct. Constructed of a special aluminum alloy that prevents corrosion and metal fatigue, is U.L. and C.U.L. 181 class 0 air duct. When properly installed only offers slightly higher air flow resistance. Can be used for heating, airconditioning, exhausting and fresh air intakes