IMC · 2021 International Mechanical Code

§403.1 · Mechanical ventilation

Effective in IMC 2021 · IMC 2024 SE Wisconsin · IMC 2024 NW Indiana.

Plain-English explainer

403.1 says where natural openings cannot meet the rate, a mechanical system delivers outdoor air at the rate set for the space type. The drafter cites it any time a permit switches a space to a bath fan, ERV, or supply run.

Why inspectors flag this

This is the rule inspectors cite when a "natural" space turns out to have no operable openings — the drafter lifts it so the contractor can add mechanical ventilation before the rough-in.

As quoted in the code: Where natural openings cannot meet the rate, a mechanical system delivers outdoor air at the rate set for the space type.

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