Flex-Vent� KP is a thermally insulated, fully lined, UL 181 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. 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 durable black polyethylene outer vapor barrier provides good airflow, thermal insulation and a double air seal. GreenGuard� Gold Certified, which is the industry�s most recognized Indoor Air Quality (IAQ) rating. The thermal application R-value is 8.0. Meets UL 181 and NFPA 90A-90B Fire Codes. Underwriters Laboratories listed for the US and Canada.