A typical pattern

Illustrative example. Not a real job or customer.

A ranch home has central air added years after it was built. On humid summer days, the cold supply ducts in the crawlspace drip onto the soil and soak their own wrap. The family notices a musty smell when the air conditioning first kicks on.

  1. A look through the hatch shows water beads on bare duct sections and darkened insulation hanging in strips.
  2. The return duct has gaps at its joints where it passes through the crawlspace.
  3. Spotting has started on the joists directly above the wettest duct runs.

The smell is a combination of moldy duct insulation and crawlspace air pulled in through leaky returns. Fixing it means dealing with the crawlspace humidity, the ducts and the growth together.

Why the ductwork matters so much

Air conditioning sends air through the ducts that is cooler than the dew point of humid crawlspace air. Any bare metal, torn wrap or crushed insulation lets that air touch the cold surface, and water condenses just as it does on a cold glass. Over a summer, that is enough to keep insulation and nearby wood damp.

Return ducts work the other way: they are under suction. A gap in a return seam inside a crawlspace pulls in air that carries soil moisture, musty odor and spores, then sends it into every room. That is why some crawlspace mold problems are noticed upstairs before anyone opens the hatch.

What the duct findings point to

  • IfInsulation wrap is wet or visibly moldyIt is usually removed and replaced after the ducts are cleaned and the crawlspace is drier.
  • IfMetal duct surfaces have growth on the outside onlyThey can usually be cleaned in place within containment.
  • IfOdor or debris comes from the registersAsk an HVAC contractor to inspect inside the duct system, and avoid running it hard until then.
  • IfReturn seams are open in the crawlspaceSeal them with mastic or approved tape as part of the repair.

Coordinating the HVAC side

  • Ask the mold contractor whether the HVAC system should be shut down during work
  • Have registers in the work area sealed during removal
  • Plan a filter change after the job, and use the best filter the system is designed for
  • Ask whether duct cleaning is needed, and by whom, rather than assuming
  • Consider older duct wrap and tape for asbestos testing before removal

General information for Wheat Ridge, not a quote or professional advice. In an emergency, call 911.