These requirements apply to materials for the fabrication of air duct and air connector systems for use in accordance with the International Mechanical Code (IMC)@ International Residential Code (IRC)@ and Uniform Mechanical Code (UMC)@ Standards of the National Fire Protection Association for the Installation of Air-Conditioning and Ventilating Systems@ NFPA No. 90A@ and the Installation of Warm Air Heating and Air-Conditioning Systems@ NFPA No. 90B. The air ducts and air connectors covered by these requirements include preformed lengths of flexible or rigid ducts@ materials in the form of boards for field fabrication of lengths of rigid ducts@ and preformed flexible air connectors. For the purpose of these requirements@ air ducts and air connectors are classified as follows: Class 0 ?C Air ducts and air connectors having surface burning characteristics of zero. Class 1 ?C Air ducts and air connectors having a flame-spread index of not over 25 without evidence of continued progressive combustion and a smoke-developed index of not over 50.