Hygrometer
Two levers, and only two
Condensation forms when a surface is at or below the dew point of the air touching it. So there are exactly two ways to prevent it:
Judging any piece of advice becomes easy once you ask which lever it pulls and how far. Sorted that way, here is what is worth doing.
Ranked by effect
| Measure | Lever | Effect |
|---|---|---|
| Replace single glazing | Surface temperature | Very large. Moves the inner pane many degrees closer to room temperature. |
| Extract at the source | Dew point | Large. Removing water before it disperses beats removing it afterwards. |
| Ventilate — when the outside is drier | Dew point | Large, but conditional. See below. |
| Raise the temperature | Surface temperature | Moderate. Warms the glass and lowers the relative humidity at the surface. |
| Move air across the glass | Surface temperature | Moderate. A stagnant film beside cold glass is the coldest air in the room. |
| Open curtains and blinds | Surface temperature | Modest but free. Closed curtains isolate the glass and make it colder. |
| Dry laundry elsewhere | Dew point | Modest to large. A load of washing is a litre or two of water. |
| Moisture-absorbing tubs | Dew point | Negligible against normal air exchange. |
| Wiping the glass | Neither | None. Protects the frame; changes nothing about tomorrow. |
The ventilation trap
"Open a window" is the standard advice and it is only half right, because ventilation swaps one air for another. It helps if the incoming air carries less water. It hurts if it carries more.
In winter that is an easy call: cold outdoor air holds very little water however high its percentage reads, so bringing it in and warming it produces dramatically drier air. Ventilating in January works.
On a mild, humid night it can be the wrong move entirely. Outdoor air at a 65°F dew point brought into a house will raise the dew point indoors, not lower it — and that is exactly the situation in opening the window for fresh air and letting the heavy air in.
The number that decides it is the outdoor dew point, which is what this site and the app show. Compare it with what you believe the indoor air is carrying; ventilate when the outdoor figure is lower.
Why the glass is the coldest thing in the room
Almost all of this is really about surface temperature, and glass loses on every count. It is thin, it conducts well, it has a large area, and on a clear night it radiates heat straight out to the sky.
Which is why the cheapest interventions are the ones that stop the pane being isolated: open the curtains, let air move past it, do not block a radiator sitting under it. A stagnant layer of air against cold glass ends up colder than the rest of the room, so it reaches its dew point first even though the room average is fine.
What is not solvable
Two cases where none of this applies:
- Water sealed between the panes — the unit has failed and is drawing in humid air. This is a glazing repair. Ventilation and heating will do nothing.
- Condensation on the outside — the outer pane has radiated heat to the night sky. It means the glazing is working, it clears by mid-morning, and there is nothing to fix.
One honest limitation
Everything above is about the air touching your glass, and this site reads outdoor air. It can tell you what the air outside is carrying, which is what decides whether ventilating will help. It cannot tell you the dew point inside a particular room — that depends on the building, the occupants and what is drying, and measuring it needs an instrument in the room. No phone has a humidity sensor.