Deep dive · Updated October 17, 2026

Preventing mold in Marin's fog-belt and shaded homes

Local versions: San Rafael, Novato, Mill Valley, Larkspur, Corte Madera, San Anselmo, Fairfax, Sausalito, Tiburon, Ross, Stinson Beach and West Marin

In many Marin neighborhoods, summer fog and winter storms mean damp air for much of the year. Here's how to keep moisture, and mold, under control.

Know your humidity

A simple hygrometer costs little. The EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent (EPA).

Ventilation

• Run bathroom fans during showers and for a while after, and make sure they vent outdoors
• Use kitchen range hoods that vent outdoors
• Vent clothes dryers outdoors

Dehumidifiers

In shaded canyon homes in Mill Valley, Larkspur and Fairfax, and fog-exposed homes in Sausalito and West Marin, dehumidifiers in damp rooms help. Empty or drain them, and clean them regularly.

Closets and exterior walls

• Leave a gap between furniture and cold exterior walls
• Don't overpack closets on exterior walls
• Consider louvered doors or leaving closet doors open

Windows

Condensation on windows is common in cold months. Wipe it up, and keep window tracks dry.

Crawlspace and attic

• Keep a sealed vapor barrier in the crawlspace (see crawlspace mold)
• Check attic ventilation (see attic mold)

Outside

• Clean gutters, extend downspouts, and slope soil away from the foundation
• Trim vegetation away from walls
• Fix roof leaks promptly (see Marin Roof Repair)

After any leak

Dry within 24–48 hours.

Understanding Marin's two damp seasons

Marin homes can be damp twice a year:

• Winter (roughly November to March): storms, cold nights, condensation on windows and in attics, and leaks
• Summer fog (roughly June to August, varying by year): marine air and low clouds, especially in southern Marin, Sausalito, the canyons of Mill Valley and the coast, keeping outdoor air damp and cool even when inland areas are hot

Many homeowners prepare for winter and forget that fog-belt homes can stay damp through summer.

A room-by-room checklist

Bathrooms - Fan vents outdoors and runs during showers plus 20–30 minutes after - Grout and caulk intact - Towels hung to dry, not piled

Kitchen - Range hood vents outdoors and is used when cooking - No leaks under the sink or dishwasher

Laundry - Dryer vents outdoors, with a clean vent line - Washer hoses in good condition, no drips

Bedrooms and closets - Furniture a few inches off exterior walls - Closets not overpacked, and doors left open sometimes - Window condensation wiped up

Living areas - A hygrometer to watch humidity - A dehumidifier where needed - Houseplants not overwatered

Crawlspace and attic - Vapor barrier intact - Attic vents clear - No bath fans venting into the attic

Choosing and using a dehumidifier

• Size it to the room and how damp it is, using manufacturer guidance
• Use a continuous drain if possible, so you don't have to empty a bucket
• Set a target humidity in line with EPA's recommendation (below 60 percent, ideally 30–50 percent; EPA)
• Clean the filter and bucket regularly, so the dehumidifier doesn't grow mold itself
• In crawlspaces, use units rated for that environment

Windows and condensation

Single-pane windows in older Marin homes sweat on cold mornings. Water runs down to the sills and can rot wood and grow mold. Wipe condensation, keep humidity in check, and consider storm windows or replacement over time. Opening window coverings during the day helps glass warm up.

Heating

Keeping the house moderately and consistently warm in winter helps reduce condensation on walls and windows. Very cold rooms that are closed off tend to get damp.

Second homes and vacation rentals

Closed-up homes, common in West Marin, can build up humidity and hide leaks. Consider a dehumidifier on a drain, a humidity sensor with remote alerts, and a periodic check by a neighbor or property manager. Our sister site's article on vacation home mold covers this.

Outdoor fixes that keep moisture out

• Gutters and downspouts: clean before the rains, extend downspouts several feet, and check for overflow during the first big storm
• Grading: soil and mulch should slope away from the foundation, not be piled against siding
• Vegetation: keep shrubs a little off the walls so siding can dry, especially on shaded sides
• Sprinklers: aim them away from walls and turn them off in the rainy season
• Decks and patios: make sure they drain away from the house

When to call a pro

If you see recurring growth, musty smells you can't trace, or humidity you can't bring down with fans and a dehumidifier, an inspection can find the source. That's often a hidden leak or a crawlspace problem.

Small habits that add up

Dry the shower after use, keep lids on simmering pots, avoid drying laundry indoors on damp days, and don't block heating registers with furniture. Each one removes a little moisture, and together they make a measurable difference in a small, damp house.

Related reading

• The complete mold guide
• Fog-belt mold article

Frequently asked questions

This guide is general information, not legal, insurance or medical advice. Sources are linked in the text. Marin Mold Removal is a referral service and doesn't perform the work.