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Apprentice answer

How do I seal a duct penetration going through a non-rated CMU wall

2nd YearField Mods, Fittings, Offsets & TransitionsCheck first

Pack the open annular space surrounding the duct tightly with mineral wool insulation backing material to fill the structural void. * Apply a thick, continuous smooth bead of standard construction polyurethane caulking or mastic sealer around both sides of the wall line to create an airtight and dust-tight perimeter wrap.

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Plain-English answer

Pack the open annular space surrounding the duct tightly with mineral wool insulation backing material to fill the structural void. * Apply a thick, continuous smooth bead of standard construction polyurethane caulking or mastic sealer around both sides of the wall line to create an airtight and dust-tight perimeter wrap.

Field checklist

Ask Foreman

Hey boss, I’m checking field mods: How do I seal a duct penetration going through a non-rated CMU wall? I found the likely issue and want to verify the next step before I lock it in. Do you want me to adjust it now or check the drawing/detail first?

Verify before acting

Use this as training guidance. The foreman, approved drawings, project specs, manufacturer installation instructions, employer safety policy, and AHJ/code requirements always control the final answer.

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