Hygrometer
First, which of the three is it?
"Condensation on windows" covers three genuinely different situations, and the fix for one is useless for the others. Establish which you have before reading anything else about it.
| Where | Cause | Can you fix it? |
|---|---|---|
| On the room side | Indoor air meeting glass colder than its dew point | Yes — raise the glass temperature or lower the air's dew point |
| On the outside | Outdoor air meeting glass that has radiated heat to a clear sky | Not really, and you would not want to — it means the glazing works |
| Between the panes | The sealed unit has failed and is admitting humid air | No. This is a glazing repair, not an air problem |
If you can wipe it off, it is the first or second. If you cannot reach it, the seal has gone and the unit needs replacing — nothing on this site will help with that, and anyone telling you to ventilate more is not answering the question you asked.
The rule behind the first two
That distinction is the whole subject. Air at 68°F with a dew point of 50°F will leave a pane at 55°F perfectly dry, even though the glass is 13 degrees colder than the room. Drop the glass to 48°F and it runs with water.
Glass is almost always the coldest surface available, because it is thin, it conducts well, and on a clear night it radiates heat straight to the sky. That is why windows show it first and walls do not.
The condensation calculator does the comparison for whatever temperatures you have.
Why some windows and not others
Once you accept it is a surface-temperature problem rather than an air problem, the pattern makes sense:
- Single glazing sits far closer to the outdoor temperature than double glazing, so it crosses the dew point on far milder nights.
- North-facing glass never gets sun to warm it.
- Behind heavy curtains the glass is cut off from the warmer air, so it runs colder still — one of the reasons closing curtains can make condensation worse rather than better.
- Rooms kept shut hold air that has nowhere to go, and unheated ones have colder glass.
What the outdoor reading tells you
Since this site reads outdoor air, it is worth being precise about what that can and cannot tell you.
It can tell you the dew point of the air outside, which is exactly what matters for condensation on the outside of the glass, for a car windscreen, for a greenhouse, and for anything else outdoors.
It cannot tell you the dew point of the air in a particular room — that depends on the building, the ventilation, how many people are in it and what is drying. No phone has a humidity sensor, and a weather service knows nothing about your house. Measuring that needs an instrument in the room.
Where it leads if left
The water is not the problem. Sustained wet surfaces are, because mould establishes above about 60% relative humidity at the surface and timber frames rot when they never fully dry.
A cold morning's condensation that clears by nine is normal and not worth acting on. Glass that is wet at noon is telling you the surface spends most of its time below the dew point, and that is worth addressing — ranked by what actually works.