I. NFPA Standard 90A, Standard for the Installation of Air Conditioning/ Ventilating Systems, 2002 Edition
This Standard states that "Class 1 rigid or flexible air ducts tested in accordance with UL 181 Standard for Safety, Factory-Made Air Ducts and Air Connectors, and installed in conformance with the conditions of listing, shall be permitted to be used for ducts when the air temperature in the duct does not exceed 121˚C (250˚F) or when used as vertical ducts up to two stories in height."
This Standard also requires that supplementary materials including duct coverings, duct linings, vapor retarder facings, adhesives, fasteners, and tapes "shall have, in the form in which they are used, a maximum flame spread index of 25 without evidence of continued progressive combustion and a maximum smoke developed index of 50 when tested in accordance with NFPA 255, Standard Method of Test of Surface Burning Characteristics of Building Materials." The Standard further requires that air duct, panel, and plenum coverings and linings "shall not flame, glow, smolder, or smoke" when tested in accordance with ASTM C 411, Standard Test Method for Hot-Surface Performance of High Temperature Thermal Insulation, "at the temperature to which they are exposed in service." In no case shall the test temperature be below 121˚C (250˚F).
The Standard defines a limited combustible material as one "that does not comply with the definition of non- combustible material that, in the form in which it is used, has a potential heat value not exceeding 8141 kJ/kg (3500 Btu/lb) (see NFPA 259, Standard Test Method for Potential Heat of Building Materials), and that complies with either of the following (a) or (b). Materials subject to increase in combustibility or flame spread rating beyond the limits herein established through the effects of age, moisture, or other atmospheric condition shall be considered combustible. (a) Materials having a structural base of noncombustible material, with a surfacing not exceeding a thickness of 3.2 mm (1/8 in.) and with a flame spread rating not greater than 50. (b) Materials in the form and thickness used, other than as described in (a), having neither a flame spread rating greater than 25 nor evidence of continued combustion and of such composition that surfaces that would be exposed by cutting through the material on any plane would have neither a flame spread rating greater than 25 nor evidence of continued progressive combustion."
II. NFPA Standard 90B, Standard for the Installation of Warm Air Heating/Air Conditioning Systems, 1999 Edition
With respect to fire safety of air duct systems, the requirements and definitions of this Standard are identical to those of NFPA Standard 90A.
These two companion standards serve as the basis for many other codes, standards and specifications. They encompass all of the fibrous glass duct insulation systems discussed herein.
III. NFPA 255, Method of Test of Surface Burning Characteristics of Building Materials
This Standard sets forth test methods for determining the surface burning characteristics of building materials, including those used in HVAC duct systems. This Standard is considered by most building code officials to be equivalent to the following:
IV. NFPA 259, Standard Test Method for Potential Heat of Building Materials
This Standard, is referenced in NFPA 90A, sets forth methods of establishing the limited combustibility classification of thermal insulating materials.