Why Ducts Under the House Get Damp
In cooling season, cold air runs through the supply ducts. When humid crawl space air touches a cold spot, such as a gap in the insulation, a torn jacket or a bare boot, water condenses on it. The EPA's mold guide says moisture control is the key to mold control and advises keeping indoor humidity below 60 percent, ideally between 30 and 50.
A humidity meter costs about $10 to $50 at a hardware store, according to the EPA, and tells you where you stand.
Rigid Metal or Flex Duct Under the House?
| Factor | Sheet metal duct | Flex duct |
|---|---|---|
| Stands up to crushing | Yes | Crushes and kinks easily |
| Cleans by source removal | Yes | Replacement is often cheaper, per the EPA |
| Needs insulation here | Yes, per ENERGY STAR | Comes insulated; the jacket must stay intact |
| Needs support | Hangers at joints | Straps so it never rests on the ground |
Torn or flattened runs go to ductwork repair before any cleaning.
Warning Signs Down There
- Standing water or damp soil under the ducts
- Insulation hanging in loose, wet or torn sections
- Musty air from floor registers when the system starts; see duct mold remediation
- Droppings, nesting material or chewed duct jackets
- Flex runs lying directly on the ground
Leave droppings and nesting material for a professional to remove.
Exhaust Rules for Crawl Spaces
Virginia's residential code keeps moist and greasy exhaust out from under the house. Bathroom exhaust must go directly outdoors, not into a crawl space (Section M1505.2), and kitchen exhaust ducts can't end in one either (Section M1503.3). If either vents under your house, fix the venting before cleaning the ducts.
Does Cleaning Crawl Space Ductwork Work?
It removes dust, debris and pest contamination from duct surfaces. It doesn't dry a damp crawl space or rescue wet insulation. The order that lasts:
Stop the Water
Drainage, vapor control and humidity first.
Replace Wet Insulation
The EPA says wet duct insulation can't be cleaned effectively.
Repair and Seal the Runs
Reconnect, support and seal the joints.
Clean the System
Now the cleaning stays clean.