Flexible duct is a non-insulated UL-181 connector. The connector is formed by encapsulating a spring steel wire helix in a tri-laminate, of a polyester, aluminum foil and polyester. These products are recognized by a “round” class 1 air connector on the product. All UL-181 class 1 air connectors are limited by NFPA 90A & 90B to a maximum length of 14’.