Symptoms
- Black or dark spots on the window sill, frame, or lower corners of the glass
- Musty smell near the window
- Wet or damp sill, especially on cold mornings
Likely causes
- CommonCondensation pooling on the sill from indoor humidityMoist indoor air condenses on cold glass and runs down to collect on the sill, feeding mold growth there.
- CommonPoor window seal or insulation making the frame run colderOlder or poorly sealed windows condense more readily than newer, well-insulated ones.
- PossibleCurtains or blinds trapping moist air against the glassReduced airflow near the window keeps humidity concentrated right where condensation forms.
What to do
- Wipe down the sill and frame each morning during periods of heavy condensation to prevent mold from establishing.
- Clean existing mold spots with a mold-specific cleaner or a mild bleach solution, then dry the area completely.
- Reduce indoor humidity with exhaust fans during cooking and showers, and consider a dehumidifier if levels stay high.
- Open curtains or blinds slightly to improve air circulation near the glass, especially at night.
- Check the window's weatherstripping and seals, replacing any that are worn or missing.
- Consider window insulation film for older single-pane windows to reduce how cold the interior glass surface gets.
- Have a window contractor assess replacement if condensation and mold are chronic despite humidity control.
Tools and parts
- Cloth
- Spray bottle
Mold and mildew cleanerSearch: “mold and mildew cleaner”
Window insulation filmSearch: “window insulation film kit”
When to call a pro
Call a window contractor if condensation and mold are chronic and tied to old or poorly sealed windows.
Safety
Use caution
Wear gloves when cleaning mold and ventilate the room if using bleach-based cleaners.