Why is there mold on my window sills every winter?

Concerns & risks

Every winter morning you open the blinds and find wet glass and little black spots creeping along the window sills. You wipe them, and a week later they're back. It looks gross and makes you worry about the air you're breathing.

Recurring mold stains frames and walls, and can bother people with allergies or asthma.

Causes & evidence

Warm, humid indoor air hits cold glass and turns into water (condensation), and mold loves that water. The science and EPA humidity targets: humidity, condensation and mold.

What to do

1. Clean the spots with detergent and water and dry the sill completely.
2. Get a cheap hygrometer. Aim for about 30–50% humidity indoors.
3. Run bathroom and kitchen fans during and after showers and cooking. Don't dry laundry indoors.
4. Open curtains in the morning and crack a window for 5–10 minutes a day.

Common mistakes: Many people just wipe the spots and repeat forever. That's fine as cleanup, but the cause is moisture, so the fix is lowering humidity. Don't block vents or keep heavy curtains pressed against cold glass all day. Avoid drying laundry indoors without ventilation. And don't paint over mold. Clean and dry the surface first. If mold covers a large area or keeps returning, consider a professional.

Speed Man tip: Mold coming back on walls too, or a big area (larger than about 3 ft by 3 ft)? Find the moisture source and consider a pro.

Wrap-up: Window mold = condensation from humid indoor air. Keep humidity around 30–50% with fans, airing out and a dehumidifier. Clean it, then fix the moisture, or it comes back.