If your windows are dripping on cold mornings, the glass is telling you something: the air inside your house holds more moisture than that cold surface can handle. Wiping it off fixes the view for a day. Lowering indoor humidity fixes the cause.
Why condensation forms on windows
Warm air holds more water vapor than cold air. When warm, humid indoor air touches glass that's been chilled by the outdoors, the air right at the surface cools below its dew point and the extra moisture turns to liquid. The colder it is outside, the colder the glass, and the lower the indoor humidity has to be to keep it dry.
A little fog on a single-pane window during a hard freeze can be normal. Water that runs down the glass and pools on the sill every morning is a sign the house is too humid, and wet sills and frames are exactly where mold likes to start.
What humidity to aim for in winter
A common rule of thumb is to lower your indoor target as it gets colder outside:
| Outdoor temperature | Indoor humidity, roughly no higher than |
|---|---|
| 40°F and up | 40–45% |
| 20°F | 35% |
| 10°F | 30% |
| 0°F | 25% |
Your exact number depends on your windows and climate. Double-pane and newer windows tolerate a bit more. The practical approach: measure, watch the glass, and adjust.
How to bring it down
- Run the bathroom fan 20 minutes after every shower. Most people turn it off when they leave the room, while the room is still full of steam.
- Use the range hood when you cook, especially when boiling water.
- Check the humidifier. A whole-house or bedroom humidifier set too high is a common culprit.
- Dry laundry in a vented dryer, not on racks indoors.
- Open curtains and blinds during the day so warm room air reaches the glass.
Measure, don't guess
You can't feel the difference between 35% and 55% humidity, but your windows can. A Wi-Fi temperature and humidity sensor shows the number on your phone and can alert you when a room drifts too high. Put one in the bedroom with the worst windows first.
If you already see mold growing on frames or walls, clean small spots and fix the moisture source. For larger areas, talk to a qualified remediation professional.