IMC · 2021 International Mechanical Code
§601.1 · Duct construction
Effective in IMC 2021 · IMC 2024 SE Wisconsin · IMC 2024 NW Indiana.
Plain-English explainer
601.1 says ducts are built from materials listed for the temperature, pressure, and exposure — galvanized steel for most supply runs, with approved alternatives for low- or high-temperature duty. The drafter cites it when a project uses flex, fiberglass, or specialty duct instead of metal.
Why inspectors flag this
A flex duct in a garage or attic that is not listed for the temperature is the classic 601.1 failure; surfacing the rule on the draft tells the contractor to swap the material before the rough-in.
As quoted in the code: Ducts are built from materials listed for the temperature, pressure, and exposure — galvanized steel for most supply runs, with approved alternatives for low- or high-temperature duty.
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