EngineerHub.orgChoose how you know the surface temperature
Measured temperature is the default because it represents the actual cold spot.
Enter the room conditions
A simple digital thermometer and hygrometer are sufficient.
Enter the coldest measured surface
Use the lowest reading from the lower glass edge, frame corner, seal or nearby reveal.
Describe the window for a rough estimate
This is less reliable than measurement, especially at frames and installation joints.
Advanced thermal-resistance input
What the temperatures mean
Worked example: why the lower window edge gets wet first
- Room air is 21°C at 55% RH.
- The calculated dew point is about 11.6°C.
- A measured frame corner is 10°C, which is about 1.6°C below the dew point.
- Condensation is therefore expected at that cold corner, even though the room-average RH is only 55%.
- The practical response is to reduce moisture, increase surface warmth or correct the cold bridge—not merely wipe the glass.
Background and recommendations
Open only the detail you need.
Dew point is the temperature at which the room’s water vapour begins to condense. A surface below this temperature can become wet.
A cold surface can have much higher local relative humidity than the room. Sustained high surface humidity can support mould on susceptible materials even before visible droplets appear.
US EPA guidance recommends keeping indoor RH below 60%, ideally around 30–50% where practical. The correct winter RH may need to be lower when windows or walls are very cold.
The estimate uses a simple one-dimensional resistance calculation. Real edge temperatures depend on spacers, frame material, installation, air leakage, curtains, radiators and thermal bridges. Use measured mode whenever possible.
Frequently asked questions
A measured temperature includes the real frame, spacer, reveal, curtains, air movement and installation effects. A U-value estimate is useful only when no surface measurement is available.
The air touching cold glass or a cold frame cools. Its local relative humidity rises and can reach saturation even when the room-average relative humidity is moderate.
No. It is a screening warning. Growth also depends on how long the surface stays damp, the material, temperature, cleanliness and whether liquid water is present.
Measure room temperature and relative humidity, then the coldest accessible glass, frame, seal or reveal temperature. Repeat during the coldest outdoor conditions.