Key takeaways
- Fog belt neighborhoods see more condensation on cold walls and windows.
- Crawl spaces and garages are frequent hidden sources.
- Bathrooms need a working fan that vents outside.
- Older homes: test for asbestos before removing moldy materials.
The fog belt effect
Neighborhoods closest to the coast — the Sunset, Richmond District, Daly City, Pacifica — spend long stretches in cool, damp marine air. Cold exterior walls and single-pane windows collect condensation, and closets on outside walls often stay damp.
Inland areas like Sacramento are drier, but summer air-conditioning and winter storms create their own moisture problems in ducts and attics.
Common hot spots
Check these places first:
- Closets and corners on exterior walls
- Window sills and frames with condensation
- Crawl spaces with blocked vents or wet soil
- Bathrooms without an exhaust fan
- Under sinks and behind washing machines
- Garage walls in older Bay Area row houses
Bay Area mold sources
Most common causes we see in older homes.
- 1Coast
Fog
Condensation on cold walls
- 2Below
Crawl space
Poor ventilation
- 3Inside
Bathroom
No exhaust fan
Prevention that works
Keep indoor humidity under about 50–60 percent, run bathroom fans during and after showers, keep crawl space vents clear, and fix leaks within a day or two.
Many fog-belt homes were built between the 1920s and 1960s, so materials around moldy areas may also contain asbestos. Test before removing them. American Heritage Environmental only tests for asbestos. We do not test for mold or lead, and we never sell or perform removal of any kind.
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