Flex-Vent KM Has A Strong Polyester Film Laminated and Encapsulating A Heavy, Corrosion Resistant Steel Wire Helix Thatforms the Innercore and Supports A Thick Blanket Of Fiberglass Insulation. The Outervapor Barrier Is A Bi-Directional, Reinforced Metallized Vapor Barrier Which Will Not Unravel. The Smooth Air Tight Inner Core Of Polyesteris Designed For Low To Medium Pressure Systems.