Metallic ducts.
Municipal Code of Chicago § 18-28-603.3
in effect18-28-603.3 Metallic ducts.
All ducts shall be constructed of iron, steel, aluminum or other approved material. Ducts shall be constructed as specified in the SMACNA HVAC Duct Construction Standards – Metal and Flexible.
Exception: Ducts installed within single dwelling units shall have a minimum thickness as specified in Table 18-28-603.3.
Table 18-28-603.3
Duct Construction: Minimum Sheetmetal Thicknesses for Single Dwelling Units
Duct Size | Galvanized Steel | Approximate Aluminum B&S Gauge
Minimum thickness (inches) | Equivalent galvanized Gauge No.
Duct Size | Galvanized Steel | Approximate Aluminum B&S Gauge
Minimum thickness (inches) | Equivalent galvanized Gauge No.
Round ducts and enclosed rectangular ducts | | |
14" or less | 0.013 | 30 | 26
Over 14" | 0.016 | 28 | 24
Exposed rectangular ducts | | |
14" or less | 0.016 | 28 | 24
Over 14" | 0.019 | 26 | 22
For SI: 1 inch = 25.4 mm
Source: City of Chicago, Municipal Code of Chicago (American Legal Publishing, codelibrary.amlegal.com)URL: https://codelibrary.amlegal.com/codes/chicago/latest/chicago_il/0-0-0-2687575Fetched: 10/24/2026