(1)
Coverings, linings and associated adhesives and insulation for air ducts, plenums and other parts of air duct systems that would have an exposed surface temperature more than 120°C when exposed to heated air or radiation from heat sources shall be of noncombustible material.
(2)
Except as permitted by Sentence (3), combustible coverings and linings, including associated adhesives and insulation, shall have
(a)
a flame-spread rating not more than 25 on any exposed surface, or any surface that would be exposed by cutting through the material in any direction, and
(b)
a smoke developed classification not more than 50.
(3)
The outer covering of ducts, plenums and other parts of air duct systems used within an assembly of combustible construction is permitted to have
(a)
an exposed surface flame-spread rating not more than 75, and
(b)
a smoke developed classification not more than 50.
(4)
Combustible coverings and linings referred to in Sentences (2) and (3) shall not flame, glow, smoulder or smoke when tested in accordance with the method of test in ASTM C411, “Standard Specification for Hot-Surface Performance of High-Temperature Thermal Insulation,” at the maximum temperature to which the coverings and linings are to be exposed in service.
(5)
Except as permitted by Sentence (6), foamed plastic insulation shall not be used as part of an air duct system or for insulating an air duct.
(6)
Foamed plastic insulation is permitted to be installed in a ceiling space that is used as a return air plenum provided the foamed plastic insulation is protected from exposure to the plenum in accordance with Article 3.1.5.15.
(7)
Combustible coverings and linings of ducts, including associated adhesives and insulation, shall be interrupted where the duct penetrates a fire separation and at the immediate area of operation of heat sources in a duct system, including electric resistance heaters or fuel-burning heaters or furnaces.