Flexible air ducts and air connectors do not have “plenum ratings” similar to those for wiring and plumbing pipe. The requirements for materials allowed within plenums is covered in NFPA Standards 90A & 90B and the applicable Mechanical Codes (IMC, UMC). UL 181 Listed and Labeled Class 0 and Class 1 Flexible Air Ducts and Air Connectors meet the requirements in these referenced standards and codes for installation within plenums. AS always, local jurisdictions have final say.