A room can have 50% relative humidity and still produce condensation on a cold window. The moisture in the air and the temperature of the glass both matter. Dew point connects the two.
For example, air at 68°F (20°C) and 50% RH has a calculated dew point of about 49°F (9.3°C). A window colder than that can collect water even though the room reading is within the EPA’s preferred humidity range. The percentage is useful, but it cannot tell you whether every surface in the room is warm enough.
Room humidity is only part of the picture
Relative humidity compares the water vapor pressure in the air with the saturation vapor pressure at the same temperature. At 50% RH, that ratio is one-half. Because saturation pressure changes with temperature, the same percentage can describe different amounts of moisture in warm and cool air. The National Physical Laboratory’s explanation sets out this relationship and the calculation behind the examples here.
Dew point expresses the same moisture condition as a temperature: the temperature to which that air would have to cool, at constant pressure, to reach saturation. The National Weather Service uses it to explain why an RH percentage needs temperature alongside it. In a room, dew point gives you a useful temperature to compare with the glass.
The air beside a cold window can cool while air farther into the room stays warmer. The room does not need to reach 100% RH everywhere for condensation to form locally. That is why a reading taken in the room and a wet patch on the window can both be telling the truth.
Change the glass temperature while keeping the room the same
In the model, set the room to 68°F (20°C) and 60% RH. Its dew point is about 53.6°F (12.0°C). Now change only the surface temperature. You are examining the same air against two different surfaces:
| Entered surface temperature | Compared with the dew point | What the example shows |
|---|---|---|
| 50°F (10°C) | About 3.6°F (2°C) below | Cold enough for liquid condensation |
| 59°F (15°C) | About 5.4°F (3°C) above | Above the condensation threshold |
Interactive illustration
When does a cold window collect water?
Change the room conditions and the glass temperature independently. The useful comparison is between the surface temperature and the air’s dew point.
Open the 3D view to rotate the model. The diagram and calculations work independently of it.
- Estimated dew point
- 12°C53.6°F
- Window surface
- 10°C50°F
- Surface comparison
- Saturation reached
Window surface is below the dew point
At 10°C, the chosen surface is colder than the air’s estimated 12°C dew point, so condensation is possible. This comparison does not predict the amount of water.
Explore the model
Room air
The air temperature and relative humidity together determine this example’s dew point. The arrows indicate air movement only; their path and speed are not calculated.
Cold window
The surface has its own temperature. A room reading does not tell you how cold the glass is. Compare the chosen surface temperature with the dew point.
Water on the glass
Below the dew point, water vapor can condense on the surface. The drawn droplets mark that condition; their number does not represent an amount of water or a drying time.
What the illustration assumes
These are entered examples, not measurements of your room or an appliance. The calculation uses a liquid-water dew-point approximation and a single nonfreezing surface. It does not model frost, airflow, drying time or building moisture sources.
The “near the dew point” band is within 1°C on either side. It is a teaching aid, not a sensor accuracy rating or a safe operating limit. “Saturation reached” describes the condensation boundary; we do not display stable surface humidity above 100%.
Changing temperature while holding RH fixed selects a new air state. The warming-only button changes both together: it assumes constant pressure with no water added or removed, and leaves the window temperature unchanged. Calculations retain the unrounded values.
Arrows, droplets, colors and animation speeds are schematic. See the NPL explanation of dew point and relative humidity.
These are calculated examples, not measured window temperatures. Entering an outdoor temperature would not tell us the temperature of the inside glass: that also depends on the window and its surroundings. The model therefore treats the surface temperature as a separate input.
Watch the difference between surface temperature and dew point. A small positive difference leaves little margin if either estimate changes. A negative difference indicates a surface below the calculated dew point; it does not predict the amount of water, the time until droplets appear or the condition of hidden building materials.
Lowering humidity and warming a surface do different jobs
Keep the room at 68°F (20°C), but reduce RH from 60% to 40%. The calculated dew point falls from about 53.6°F to 42.8°F (12.0°C to 6.0°C). A surface that was below the first dew point may be above the second. Removing water vapor has changed the threshold itself.
Warming the surface works differently. The dew point of the room air can stay the same while the glass moves above it. EPA moisture-control guidance recognizes both approaches: reduce moisture or increase the temperature of a cold surface. Which is appropriate in a particular home depends on why the surface is cold and where the moisture comes from.
This is why repeatedly lowering a dehumidifier setting may be an incomplete response. A persistent cold spot, inadequate air circulation, a leak or another moisture source may need attention of its own. The calculation helps separate questions; it cannot decide which building repair is needed.
Heating can lower the reading without removing water
Imagine air at 64.4°F (18°C) and 60% RH warming to 68°F (20°C), with no moisture added or removed and pressure held constant. Its RH falls to about 53%, while its dew point stays about 50.2°F (10.1°C). The air has not been dehumidified.
ASHRAE’s treatment of sensible heating describes this as a change in temperature at the same humidity ratio—the same mass of vapor per mass of dry air. A heater may also warm nearby surfaces, which can help with condensation, but the lower RH number alone does not prove that water has left the room.
There is a related trap in calculators: changing the air temperature while keeping RH fixed describes a different moisture state. It is not the same experiment as warming the original air without moisture exchange. Keep track of which quantity you are holding steady.
Use the result alongside observations of the room
The EPA recommends indoor RH below 60%, ideally 30–50% where possible. That remains useful general guidance. It is not a promise that a sufficiently cold window will stay dry, nor does a clear window establish that a building is free of moisture problems.
This illustration concerns liquid condensation on nonfreezing surfaces. It does not model frost, drying inside walls or mold growth. Fix leaks and investigate persistent wetness; existing mold requires the separate attention described in EPA guidance. For everyday appliance targets, use the humidity-setting guide. If you are considering airing the room, compare indoor and outdoor dew points before assuming that the lower RH percentage identifies the drier air.
Sources
Checked November 1, 2026. Calculations and illustrations explain physical relationships; they are not measurements of an appliance or home.
Related guidance
Spotted an outdated specification or unclear explanation? Send a correction with the source.