Designed for high pressure applications, these air duct connectors come with a double-ply construction utilizing metalized/polyester laminate, encapsulating a high carbon steel wire helix for strength and durability. Concerned about code approvals? These flexible duct products meet or exceed the requirements of most federal, state, and local mechanical and energy codes. The flexible duct complies with The Standard Mechanical Code (SBCCI), the National Mechanical Code (BOCA), the Uniform Mechanical Code (ICBO), NFPA 90A and 90B, HUD, and the Florida Energy Code, just to name a few. Be certain that the duct you are buying meets these stringent codes. UL-LISTED. These flexible air duct connectors have been rigorously tested by Underwriters Laboratories, Inc. in accordance with UL 181, The Standard for Safety for Factory-Made Air Ducts and Air Connectors. UL is a not-for-profit laboratory with an outstanding reputation for fire safety and performance testing. Make sure that the flexible duct you buy bears this UL mark.