Flex-Vent� KM is a thermally insulated, fully lined, UL 181 and CUL S110 Class 1 flexible air duct designed for low to medium pressure cooling and heating HVAC systems. It can be used in either supply or return sections and provides economical means for handling misalignment between systems components and ducting around obstacles where fabricated and fitted ducts are difficult and costly to use. It is equally suitable for new jobs or retrofit work and is especially useful when making system extensions or for changing conditioned zones, now or in the future. Compliance to NFPA Standards allows you to install lengths longer than the 14' limitation applying to connectors. It is a highly efficient part of the total duct system because the smooth liner, the ample insulation and tough outer vapor barrier provide good airflow, thermal insulation and a double air seal.