IMC · 2021 International Mechanical Code

§603.4 · Insulation

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

Plain-English explainer

603.4 (Table 603.4) sets supply-duct insulation values: R-6 flexible / R-8 rigid for supply, R-6 for return. The drafter cites it for any permit that runs new supply trunks through an unconditioned attic, garage, or crawlspace.

Why inspectors flag this

Inspectors carry a tape and a caliper for 603.4 — wrong R-value on the supply run is a top-five rough-in flag; the drafter lifts the rule so the wrap matches before the inspection.

As quoted in the code: Supply ducts in unconditioned space meet a minimum R-value per Table 603.4 (R-6 flexible or R-8 rigid for supply; R-6 for return).

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