Duct is sweating or condensation is showing after startup. What do I check?

Treat sweating duct as an insulation/vapor-barrier and operating-condition problem. Look for missing insulation, open seams, crushed wrap, torn vapor barrier, exposed cold metal, bad joints, or unusual humidity/temperature conditions.

Check insulation continuity, vapor barrier, penetrations, damage, and operating conditions before patching or blaming equipment.

Equipment Connections: RTU, AHU, VAV, Fans & Louvers 2nd Year YELLOW

Watch out

Do not cover wet insulation, ignore mold/ceiling damage risk, or patch vapor barrier without verifying the cause.

Check

Duct system type
Where moisture starts and stops
Insulation/vapor barrier continuity
Hanger/saddle/penetration damage
Room humidity or special environment

Steps

Do not wipe it and walk away.
Identify if the duct is supply, return, outside air, or special environment.
Check insulation continuity, vapor barrier seams, penetrations, hangers, saddles, and damaged wrap.
Photograph wet spots and surrounding conditions.
Ask before cutting insulation, sealing, or blaming the unit.

Say this to your foreman

This duct is sweating at [location/system]. I checked insulation continuity and vapor barrier damage. Do you want me to repair wrap, check operating conditions, or hold for startup/TAB direction?

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