Find your dew point and condensation risk
Air at 70°F and 50% humidity has a dew point of about 50°F. Any surface in that room colder than 50°F will sweat. Window glass on a winter night, a closet on an exterior wall, the corner behind a dresser. That's where condensation mold starts. Enter your numbers and see where your house stands.
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Surface temperatures for windows and walls are estimates from typical construction. A cheap infrared thermometer gives you the real number.
Dew point quick reference (70°F room)
| Indoor humidity | Dew point | What sweats |
|---|---|---|
| 30% | 37°F | Single-pane glass on a very cold night |
| 40% | 45°F | Single-pane glass below freezing outside |
| 50% | 50°F | Single-pane glass most winter nights, double-pane on cold nights |
| 60% | 55°F | Double-pane glass, cold corners, basement concrete in summer |
| 70% | 60°F | Almost any exterior surface in winter, AC ducts in summer |
This is why the standard advice is 30 to 50% indoor humidity. Above that, ordinary surfaces in ordinary houses start hitting the dew point.
Frequently asked questions
What is dew point in plain terms?
The temperature at which the air can't hold its water anymore. Cool air to its dew point and the water comes out as fog, dew, or drops on a cold surface. A higher dew point means more water in the air, whatever the humidity reading says.
Why do my windows sweat in winter?
Glass is the coldest surface in the room. Indoor air touching it cools to the glass temperature, and if that is below the dew point, water forms. Lower the humidity or warm the glass. Both work.
Does condensation cause mold?
Yes, and it's one of the most common causes people miss because there is no leak. Window sills, the bottom of exterior walls, closet corners, and behind headboards on outside walls all get mold from repeated condensation.
How is this different from the humidity calculator?
The humidity calculator looks at the room as a whole. This one looks at a specific cold surface, which is where mold actually starts. Use both.
How accurate is the surface temperature estimate?
It uses typical values for each construction type. Real numbers vary with wind, curtains, and how well the wall was built. An infrared thermometer costs about twenty dollars and gives you the exact reading. Pick "I measured the surface temperature" to use it.
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