Where the water comes from
CDPH's list of outdoor moisture sources reads like a crawlspace checklist: flooding, ground that slopes toward the house, and downspouts and sprinklers aimed at the house (CDPH). Add plumbing leaks under the floor and high groundwater during long storm cycles, and many Sonoma County crawlspaces get wet at least some winters.
How to check yours
After a big rain, open the crawlspace access with a strong flashlight. Don't go in if there's standing water near wiring, or if you're not comfortable in tight spaces. Look for:
- Standing water, mud or water lines on the foundation
- Wet or fallen insulation
- Growth on joists, beams and subfloor
- Damp soil and a musty smell
- Disconnected or dripping pipes and ducts
Fixes, from simplest to biggest
- Gutters and downspouts: clean them and extend downspouts well away from the foundation.
- Grading: slope soil away from the house where possible.
- Plumbing repairs under the floor.
- Vapor barrier: a plastic ground cover reduces moisture rising from soil.
- Drainage and sump pump for crawlspaces that take on water.
- Encapsulation and dehumidification in some homes, sealing the space and controlling humidity. The EPA's indoor target is below 60 percent, ideally 30 to 50 percent (EPA).
Want a local pro to take a look?It's free for homeowners and there's no obligation.
When there's already mold
Wet insulation usually comes out. Framing is cleaned, dried and verified before anything is closed up. If floodwater got under the house, treat it as contaminated (EPA). See crawlspace mold removal.
West county, river and Petaluma homes
Shaded lots in Sebastopol and Forestville, raised homes on the lower Russian River and older homes near the Petaluma River tend to have the toughest crawlspaces. If your area floods, also read El Niño flood prevention on our sister site.
Sources
- CDPH: Frequently asked questions about mold
- EPA: A Brief Guide to Mold, Moisture and Your Home
- EPA: Mold cleanup in your home
Sources checked October 18, 2026.
Frequently asked questions
Is some moisture in a crawlspace normal?
Some humidity is normal, but standing water, wet insulation and visible growth aren't.
Should crawlspace vents be closed in winter?
It depends on the home's design. Closing vents without a moisture plan can make things worse. Ask a provider to assess your specific crawlspace.