Fire Resistance-Rated Systems are evaluated under the ANSI/UL 263 Standard “Fire Tests of Building Construction and Materials.” A complete list of Fire Resistance-Rated Systems can be found in UL’s On-Line Certifications Directory. By entering the System “design number” you can determine what materials have been tested as part of the system and are allowed to be used as air ducts in that rated system. In many Fire Resistance-Rated Systems, Class 0 or Class 1 Flexible Air Ducts are allowed to be used. But this may not always be the case. The designer and installer must verify by individual design number what duct materials are allowed within that particular rated system.