Flexible Duct Flex Duct 12 inchx25 feet. Code Compliance: UL 181, SBCC, BOCA, NFPA 90A and 90B and HUD 3280.715(b)A double lamination of tough polyester which encapsulates a steel wire helix forms the air-tight inner core. The smooth inner core and fiberglass insulation are reinforced with scrim, and sheathed in a black, heavy-duty polyethylene jacket which complies with the HUD requirements for outdoor mobile home use.The product is designed to be used as: a supply and return duct with self-contained A/C, air duct system and or branch duct with upflow or downflow HVAC. R - Rating: 4.2. Thickness 1-1/4 inch.