Hygrometer
Not the inside. The outside.
It is July, the house is air conditioned, and at half past six in the morning the front windows are running with water on the outer surface. Every instinct says something has failed — a seal, a gutter, something in the wall.
Nothing has failed. This is the glazing working.
What is happening
The outer pane has spent the night radiating heat to the sky and has ended up several degrees colder than the outdoor air. When it fell below the dew point of that humid summer air, water condensed onto it.
It is dew. The same dew that is on the lawn and the car, forming on a vertical pane instead.
Why it means the windows are good
This is the counterintuitive part, and it is worth stating plainly.
For the outer pane to get that cold, it has to be thermally disconnected from the warm building behind it. That is exactly the job of a modern sealed unit: a low-emissivity coating and an insulating gas fill mean very little heat escapes to warm the outer glass.
Old single glazing almost never does this, because heat pouring out of the house keeps the outside of the pane warm all night.
| Glazing | Outer pane at dawn | Outside condensation |
|---|---|---|
| Single | Warmed by heat escaping the house | Rare |
| Older double | Slightly warmed from inside | Occasional |
| Modern low-E unit | Tracks the outdoor air, then goes below it | Common on clear nights |
So the appearance of this after new windows are fitted is not a coincidence and not a defect. It is the most visible evidence you will get that they are performing.
Why it is summer
It needs a high dew point. The glass can only cool a few degrees below the air, so the air has to be close to saturation already for that to be enough.
Summer nights supply it: humid air, clear skies, and long hours of radiative cooling once the sun has gone. In winter the air is far drier — the dew point is a long way below the temperature — so the pane never gets close.
The air conditioning contributes indirectly, by keeping the interior from warming the glass. But the main actor is the sky.
The three conditions
All three at once, which is why it is not every night:
- A humid night, so the dew point spread is small.
- A clear sky, so the glass can radiate freely. Cloud cancels it.
- Still air, so warmer air is not being mixed across the surface.
When it clears, and what to do
Within an hour or two of the sun reaching the glass. Nothing needs doing — as the pane warms past the dew point, the water evaporates back into the air it came from.
The only case worth acting on is water sealed between the panes, which you cannot wipe off. That is a failed unit and a glazing repair, and it has nothing to do with the weather.
The same effect, everywhere else
Once you recognise radiative cooling you see it constantly: frost on a car roof when the air stayed above freezing, dew on the lawn after a dry week, a tent raining inside, skylights fogging first.
All the same thing — a surface that got colder than the air, and air that had water to give.