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How cold does a single glazed window get, and will it condense?

Work out the dew point in a room, whether a cold window or wall will run with water, and the humidity you need to stay below to keep it dry.

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Single glazing on a cold night sits only a few degrees above the outside air, which is why it condenses at humidity levels double glazing shrugs off.

Rates vary widely across Canadian — picking yours makes the answer far closer.
The room
°C
The air temperature in the middle of the room, not against the window.
%
From a hygrometer — a basic one costs very little and is the only way to know this rather than guess. UK homes commonly sit at 50 to 70% in winter.
The cold surface
°C
The window pane, wall or corner you are worried about. A cheap infrared thermometer reads it in a second; single glazing on a cold night can sit near freezing.
On that surface Condensation will form
Dew point (°C) 10.7
Keep humidity below 40%
Humidity at the surface 100%
Mould risk Wet surface — mould very likely
Margin above dew point (°C) -4.7

Common cases

What drives the cost

Single glazing has almost no insulating value, so the inside face sits only a few degrees above the outside air. On a freezing night that puts it near zero — far below the dew point of any normally occupied room, which is why single glazed windows run with water at humidity levels double glazing handles without a mark.

It also means the humidity ceiling is brutally low. Where a double glazed pane might tolerate 60% room humidity, single glazing on the same night can need you below 35% — a level that is neither comfortable nor realistically achievable. Secondary glazing or a thermal blind raises the glass temperature and does more than any amount of wiping.

How this is worked out

Warm air holds more moisture than cold air. The dew point is the temperature at which the air in your room would be completely full — so any surface colder than the dew point pulls moisture out of the air and gets wet. That is all condensation is. The calculation is the Magnus formula, which relates temperature and relative humidity to the dew point, and it is pure physics: nothing about it varies by country, supplier or season.

What this assumes

  • The room is at one temperature and humidity throughout. In reality both vary — it is colder and damper near an external wall than in the middle of the room, which is exactly why corners go mouldy first.
  • The surface temperature is the one you measure. Behind furniture or inside a corner it is usually a degree or two colder than the open wall beside it, and that difference is often the whole problem.
  • Mould is treated as a risk above about 80% humidity at the surface, which is the widely used threshold for germination. It also needs time — hours a day, over weeks — so brief peaks after a shower matter far less than a wall that sits damp.
  • Nothing here accounts for how the moisture got into the air. Cooking, showering, drying clothes and breathing all add it; ventilation is what takes it out.

Questions

Do those plastic film secondary glazing kits work?

Better than people expect for this specific problem. They trap a layer of still air against the glass, which raises the temperature of the inner surface you actually see — and it is that surface temperature, not the window's overall performance, that determines whether it condenses.

Why does condensation appear on the window and not the wall?

Because glass is usually the coldest surface in the room. It has far less insulation than a wall, so it drops closest to the outside temperature and reaches the dew point first. That also makes a window a useful early warning: if it is streaming, the cold corners of your walls are not far behind.

Is it better to turn the heating up or open a window?

Both help, for different reasons. Heating raises the surface temperature so it sits further above the dew point; ventilating removes the moisture so the dew point itself falls. Ventilating is usually the cheaper of the two, because you are removing the cause rather than compensating for it.

Why does mould grow where there is no visible water?

Because mould does not need liquid water, only high humidity at the surface — around 80% is enough. A wall can sit a degree or two above the dew point, look and feel completely dry, and still be damp enough for mould to establish over weeks.

Does a dehumidifier fix this?

It treats the symptom effectively, by lowering the room humidity and so the dew point. It costs electricity to run, and it does nothing about a cold surface, so a badly insulated wall will still be the coldest thing in the room. Ventilation and insulation address the cause; a dehumidifier manages it.

Related tools

Sources

Rates from Hydro-Québec for rates in effect 1 April 2025. Last reviewed August 21, 2026.