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Apprentice Q&A · #099

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

2nd YearField ModsYELLOW · Check 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?

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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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Related Field Rescue routes

What do I do before insulating or covering an S-and-drive joint?Field Rescue · Rectangular Duct Assembly: S-and-DriveWhat do I do when not using UL 181-rated material where required?Field Rescue · Sealing, Leakage, Mastic & Pressure TestingWhat do I do when insulation left open at wall penetration?Field Rescue · Insulation, Liner, Vapor Barrier & Condensation

Related mistakes & recovery checks

#074 · Forgetting to install a fire damper in a rated wall penetration2nd Year · Installation#053 · Forgetting to allow for duct insulation thickness when calculating clearances2nd Year · Measuring & Layout#063 · Letting the bead roller drift off-line because feed pressure was uneven2nd Year · Tools & Equipment