Is mold on window sills dangerous?

Why window sills grow mold

Windows are usually the coldest surfaces in a room. When indoor air is humid, water condenses on the glass, runs down and collects on the sill and in the frame. Add a little dust and a wooden or painted sill, and mold has what it needs. That is why the problem tends to show up in bedrooms, where people breathe all night with the door closed, in bathrooms and kitchens, and in cold months when the temperature difference is greatest.

Window tracks on sliding windows and the bottom rail of storm windows collect water and dirt the same way, and they are easy to miss. Weep holes in the frame, which are meant to let water drain outside, can clog with dirt and hold water in the track. Clearing them with a thin wire or a small brush is a simple fix that many people don’t know about.

Cleaning mold off a window sill

A small patch on a sill falls well under the roughly 10 square feet the EPA suggests homeowners can generally handle themselves.

1. Put on an N95 respirator, gloves and goggles, and open the window if the weather allows.
2. Wipe the growth with detergent and warm water, working from clean areas toward the moldy spot.
3. Use a soft brush or old toothbrush for corners and tracks.
4. Wipe again with a clean damp cloth, then dry the sill and frame completely.
5. Bag the cloths and wash your clothes afterward.

Skip bleach on wooden sills. It can discolor the wood and adds water to a porous surface. Our answer on whether vinegar kills mold covers when a milder cleaner is reasonable.

Keeping it from coming back

Cause	What helps
Humid indoor air	Keep humidity below 60 percent, ideally 30 to 50 percent as the EPA recommends; lower in very cold weather
Blocked airflow	Open curtains and blinds during the day so room air reaches the glass
Moisture from showers and cooking	Run exhaust fans vented outdoors
Drying laundry indoors or an overly high humidifier setting	Dry clothes elsewhere, turn the humidifier down
Old or single-pane windows	Storm windows, weatherstripping or replacement over time

A quick daily wipe of condensation on the coldest mornings also makes a big difference. Our answer on whether mold can grow in cold weather goes deeper into winter condensation.

When to look further

Most sill mold stays a cleaning chore. It is worth a closer look when:

• The wood of the sill or frame is soft, dark or crumbling, which suggests rot from long-term water
• Staining runs down the wall below the window or appears on the drywall around it
• Water appears after rain rather than on cold mornings, which points to a leak around the window rather than condensation
• Several windows in the house have growth despite regular cleaning
• There is a musty smell in the room that doesn’t go away

A leaking window can wet the framing and insulation inside the wall, well away from the sill you can see. That kind of problem is found with moisture readings rather than cleaning, and our knowledge page on mold inside walls explains how.

We remove mold and mycotoxins from buildings. We don’t diagnose or treat medical conditions. If you or anyone in your home has symptoms, please talk with a doctor. With your permission, we’re glad to share test results with them.