Fiberglass duct board is a common choice for residential HVAC systems in Florida because it combines insulation and duct structure in one easy-to-install product. But while it works well for thermal control, its porous core can trap moisture if leaks, condensation, or improper airflow occur.
Unlike metal ductwork, once fiberglass gets damp inside, it’s nearly impossible to dry out completely. That’s why hidden mold inside duct board is one of the most common surprises during home inspections — especially in older homes or those with a history of humidity problems.
If you suspect your duct board may be holding moisture, here’s how an inspection typically works, what you can check yourself, and when to call in a professional.
✅ 1️⃣ Start with What You Can See and Smell
Before cutting open ducts or hiring a specialist, there are signs you can pick up without tools:
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Musty Odors: If you notice a stale, musty smell when the AC cycles on, it could point to moisture or microbial growth inside the duct liner.
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Visible Mold or Staining: Check around supply registers and return grills. Black streaks, dark spots, or water stains near vents can indicate moisture traveling from deeper inside.
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Condensation in the Attic: If you have access to the attic, inspect the outer surface of the duct board. Beaded water, rust on metal connections, or insulation that feels damp are all red flags.
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Uneven Cooling or Humidity: Rooms that feel clammy or fail to cool evenly may signal duct leaks or hidden blockages due to deteriorating liner.
✅ 2️⃣ Check the Air Handler and Drain Pan
One of the most common causes of moisture inside duct board is poor drainage at the air handler:
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Inspect the drain pan for standing water.
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Ensure drain lines are clear and properly pitched.
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Look for signs of rust or water overflow.
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Confirm there are no air leaks where the supply plenum connects to the air handler.
Moisture at the air handler can easily travel into the duct board supply trunk and get absorbed into the liner.
✅ 3️⃣ Inspect Inside (When Safe and Practical)
Most homeowners won’t cut into their own ductwork — nor should they. But in some cases, visible access is possible through supply registers or service panels:
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Remove a supply register and use a flashlight to look inside.
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Look for dark patches, mold growth, or wet spots along the liner.
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A musty smell near the opening is also a clue.
Never disturb moldy duct liner yourself — fiberglass fibers can break loose, and disturbing mold can spread spores further.
✅ 4️⃣ Use Professional Inspection Tools
Certified air quality inspectors and HVAC professionals use:
- Borescopes or inspection cameras: Small flexible cameras can snake deep into duct runs to check for damp liner, visible mold, or failing seams.
- Moisture meters: Surface and pin-type moisture meters check duct liner saturation at accessible points.
- Air sampling: If mold is suspected but not visible, air samples (spore traps) can measure what’s circulating through the system and confirm hidden microbial growth.
- Manometers: Technicians may also use manometers to measure static pressure inside the duct system. Abnormal pressure readings can indicate air leaks, blockages, or airflow imbalances that may be contributing to condensation and hidden moisture buildup inside the liner.
✅ 5️⃣ What Happens If Moisture or Mold Is Found?
Once moisture is trapped inside fiberglass duct board, cleaning is rarely enough. The fibrous core can’t be thoroughly dried or scrubbed without damaging its structure.
Common solutions include:
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Partial duct board replacement for affected sections.
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Upgrading to new duct board or metal duct with external insulation.
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Balancing the HVAC system to correct airflow and temperature issues.
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Fixing drain pans, leaks, or insulation gaps that caused the problem.
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Post-remediation air sampling to verify mold clearance.