Storm-season article · Updated October 18, 2026

Winter condensation and mold: how to control humidity at home

Not all mold comes from leaks. In winter, warm, moist indoor air hits cold windows and exterior walls, and the water it leaves behind can feed mold on sills, frames and closet walls. CDPH lists condensation on cold surfaces as a common indoor moisture source (CDPH).

Signs of a condensation problem

• Windows that fog or drip on cold mornings
• Black spots on window sills, frames or caulk
• Mold on the backs of furniture against exterior walls
• Damp closets on outside walls
• Musty bathrooms

Fixes that work

• Target humidity: the EPA recommends keeping indoor humidity below 60 percent, ideally 30 to 50 percent (EPA). An inexpensive hygrometer tells you where you are.
• Run exhaust fans during and after showers and cooking, and make sure they vent outdoors. CDPH lists poor venting in bathrooms, laundry areas and kitchens as a moisture source.
• Vent the dryer outside.
• Use a dehumidifier in damp rooms.
• Keep air moving: pull furniture a few inches off exterior walls and leave closet doors open sometimes.
• Wipe condensation off windows in the morning.

Cleaning small spots

Scrub hard surfaces with detergent and water and dry them (EPA). CDPH doesn't recommend bleach. Don't paint or caulk over mold. If it keeps coming back or covers a large area, it's time for a mold inspection.

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.

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