Dew Point Calculator · Practical guide
Why Is There Condensation on My Windows?
Window condensation means the glass is colder than your indoor dew point — the fix is warmer glass or drier air.
At a glance
Quick answer
Condensation forms on windows when the glass surface cools below your home's dew point — the temperature at which indoor air can no longer hold its moisture. Room air at 70°F and 50% humidity has a dew point of about 50.5°F, so any glass colder than roughly 50°F will fog or drip on a cold night.
Interactive tool
Dew Point Calculator
Enter your own values, then compare the result with the examples below.
Dew Point Results
The Dew Point Connection
Window glass is usually the coldest surface in a room because it faces the outdoors. When humid indoor air touches glass below the dew point, water vapor condenses into droplets — exactly like a cold drink glass sweating in summer. Use the calculator above with your room temperature and a humidity reading to find the exact threshold your windows must stay above.
Example Thresholds
| Room conditions | Dew point | Condensation risk |
|---|---|---|
| 70°F, 40% RH | 44.6°F | Low — most double-pane glass stays above this |
| 70°F, 50% RH | 50.5°F | Moderate — cold nights may fog single-pane glass |
| 70°F, 60% RH | 55.5°F | High — even double-pane windows can sweat |
How to Stop It
- Lower indoor humidity: run exhaust fans when cooking and showering, and vent dryers outside.
- Warm the glass: better-insulated windows, storm windows, or thermal curtains raise the surface temperature.
- Move air across the glass with a fan so the surface layer does not cool to the dew point.
When to Worry
Persistent condensation can feed mold on sills and frames. Condensation between the panes of a double-pane window means the seal has failed and the unit needs replacement — no humidity fix will help that one.
Frequently Asked Questions
Is window condensation a sign my windows are bad?
Not necessarily. Interior condensation usually means indoor humidity is high relative to how cold the glass gets. Only moisture between the panes points to a failed window seal.
What indoor humidity prevents window condensation?
In winter, keeping indoor relative humidity around 30-40% usually keeps the dew point below the temperature of double-pane glass. Check your room's dew point above and compare it to how cold your windows get overnight.
Why do bedroom windows fog up overnight?
Two sleeping people add moisture to the air all night while the glass cools steadily. The dew point rises as the glass temperature falls, and they meet around early morning.
Does a dehumidifier stop window condensation?
Yes, by lowering the dew point of the room air. Dropping humidity from 60% to 40% at 70°F lowers the dew point from 55.5°F to 44.6°F — often enough to clear the glass completely.