MoldHelpUSA
Mold on a window sill grows where water collects and stays. Most of the time that water is condensation running down cold glass, or rain finding its way in around the frame. Dust, pollen, and dirt on the sill give the mold something to feed on. Small spots on a vinyl or painted sill are easy to clean. Mold that keeps coming back, or that has spread into the wood or the wall below, means the water source has not been fixed.
Why mold grows on window sills
Condensation in cold weather
When warm, humid indoor air touches cold glass and frames, water condenses and runs down onto the sill. This is most common in winter in colder places such as Washington, DC, Denver, and Colorado Springs, especially in bedrooms and bathrooms, behind closed curtains, and on older single-pane or aluminum windows.
Leaks around the frame
In warm, rainy places such as Florida and the Gulf coast, the more common cause is water getting in around the window: failed caulk, wind-driven rain, clogged weep holes in the bottom of sliding and vinyl frames, or a flashing problem behind the stucco or siding. Older windows in block homes in Miami and West Palm Beach and storm-driven rain in Houston are typical examples.
High indoor humidity
Cooking, showers, drying laundry indoors, and closed-up rooms all raise humidity. The EPA recommends keeping indoor humidity below 60 percent, ideally 30 to 50 percent. Above that, sills, frames, and corners stay damp.
Black, white, or green: does the color matter?
Not for cleaning. Dark spots are the most common and are not necessarily the mold called black mold; see what black mold looks like. White or gray fuzz is also common on wood sills; see white mold. The method is the same whatever the color.
How to clean mold off a window sill
Wear gloves and goggles, and an N-95 respirator if there is more than a few small spots. Open the window if the weather allows.
| Sill or frame | How to clean it |
|---|---|
| Vinyl or aluminum | Wipe with detergent and water, scrub the corners and tracks with a soft brush, rinse, and dry. Clear the weep holes with a thin wire. |
| Painted wood, firm | Wipe with detergent and water, dry thoroughly, then repaint or seal once dry. |
| Bare or stained wood, firm | Clean, let it dry completely, lightly sand stained areas while wearing a respirator, then seal or paint. |
| Soft, spongy, or crumbling wood | The wood is decaying. It needs to be repaired or replaced, and the source of the water found. |
| Drywall or plaster below the window | Light surface spots on painted drywall can be wiped. Stained, soft, or bubbling drywall usually has to be cut out. See mold on walls. |
| Rubber gaskets and seals | Wipe with detergent and water; replace gaskets that stay stained or have cracked. |
Do not paint or caulk over mold. The EPA advises against it, because paint over mold is likely to peel and the growth can continue underneath.
Bleach, vinegar, and other products
The EPA does not recommend bleach or other biocides as a routine part of mold cleanup. On hard, non-porous surfaces such as vinyl, aluminum, and glass, detergent and water are enough. On wood, bleach does not reach below the surface, and dead mold can still cause allergic reactions, so physical cleaning and drying matter more than the product. Never mix bleach with ammonia, vinegar, or other cleaners.
How to keep mold off your window sills
- Lower the humidity. Aim for 30 to 50 percent. Run the air conditioning or a dehumidifier in humid weather.
- Use exhaust fans when showering and cooking, and vent the dryer outside.
- Let air reach the glass. Open curtains and blinds during the day, and do not push furniture against windows.
- Wipe condensation off the glass and sill in the morning during cold weather.
- Keep sills and tracks clean, so there is less for mold to feed on.
- Maintain the frame. Replace failed caulk, keep weep holes clear, and have damaged flashing or failed window seals repaired.
When to call a professional
- The mold returns within weeks after cleaning
- The sill or frame is soft or rotted
- The wall below the window is stained, bubbling, or soft
- You see growth behind the trim or the smell is strong near the window
- More than about 10 square feet is affected, the EPA's threshold for professional work
Those signs suggest water is getting into the wall, not just onto the sill. A local inspector can check the wall below the window with a moisture meter, and the call to find one is free. See mold behind drywall.
Health
Condensation mold is most common on bedroom windows, which means people spend the night breathing the air next to it. According to the CDC, mold can cause a stuffy nose, sore throat, coughing or wheezing, burning eyes, or skin rash in sensitive people, and people with asthma or mold allergies may react more strongly. If someone wakes up congested or wheezy in one room more than others, keep the sill clean and dry, and talk to a doctor about the symptoms.
See also musty smell in the house and mold in the closet.
Next step
A sill you wipe every week is a maintenance chore. A sill that turns soft, or a wall under the window that stains and bubbles, is a water problem inside the wall. For a homeowner, that is the point to call the number on this page: the free call connects you with an independent local mold inspection company to find where the water enters, or a mold remediation company if wood or drywall already has to come out.