Flex Duct - UL 181 Insulated, Class 1, 25 Feet per BoxCLICK HERE FOR SUBMITTAL Our premium insulated flex duct is UL 181 compliant, Class 1 Air Ducts and are manufactured with a tri directional fiberglass scrim reinforced, metallized polyester outer jacket. A double lamination of tough polyester which encapsulates a steel wire helix forms the air tight inner core of these flex ducts. The double layer core of each product is wrapped with a thick blanket of fiberglass insulation. The inner core of our