A double lamination of tough polyester which encapsulates a steel wire helix forms the air-tight inner core. The double-layer core is wrapped in a thick blanket of fiberglass insulation and sheathed in a rugged and durable reinforced polyester jacket. Designed for indoor use as a supply and return air duct in residential and commercial low-to-medium pressure heating and air conditioning systems. UL Listed 181, SBCC, BOCA, NFPA 90A and 90B, HUD. Meets most federal, state and local codes.We recommend that you check with the local code body having jurisdiction in your area to determine applicable codes.