- Equipment connected to duct systems shall be designed to limit discharge air temperature to a maximum of 250°F (121°C).
- Factory-made air ducts shall be constructed of Class 0 or Class 1 materials as designated in Table M1601.1.1(1).
- Fibrous duct construction shall conform to the SMACNA Fibrous Glass Duct Construction Standards or NAIMA Fibrous Glass Duct Construction Standards.
- Minimum thickness of metal duct material shall be as listed in Table M1601.1.1(2). Galvanized steel shall conform to ASTM A 653. Metallic ducts shall be fabricated in accordance with SMACNA Duct Construction Standards Metal and Flexible.
- Use of gypsum products to construct return air ducts or plenums is permitted, provided that the air temperature does not exceed 125°F (52°C) and exposed surfaces are not subject to condensation.
- Duct systems shall be constructed of materials having a flame spread index not greater than 200.
- Stud wall cavities and the spaces between solid floor joists to be used as air plenums shall comply with the following conditions:
- These cavities or spaces shall not be used as a plenum for supply air.
- These cavities or spaces shall not be part of a required fire-resistance-rated assembly.
- Stud wall cavities shall not convey air from more than one floor level.
- Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fireblocking in accordance with Section R602.8.
- Stud wall cavities in the outside walls of building envelope assemblies shall not be utilized as air plenums.
|DUCT CLASS||MAXIMUM FLAME SPREAD INDEX|
|Minimum Thickness |
|Equivalent Galvanized |
|Minimum Thickness |
|Round ducts and enclosed rectangular ducts |
14 inches or less
16 and 18 inches
20 inches and over
|Exposed rectangular ducts |
14 inches or
Over 14a inches
Exception: Spray application of polyurethane foam to the exterior of ducts in attics and crawl spaces shall be permitted subject to all of the following:
Exception: Dedicated forced-air systems serving only a garage shall not be prohibited from obtaining return air from the garage.