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Why does the bathroom mirror and window stream after a shower?

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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A warm bathroom immediately after use, when humidity is at its highest and every cool surface is below the dew point.

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) 19.4
Keep humidity below 65%
Humidity at the surface 100%
Mould risk Wet surface — mould very likely
Margin above dew point (°C) -4.4

Common cases

What drives the cost

A hot shower can push a small bathroom close to saturation in minutes, so essentially every surface below the air temperature goes wet. This one is not a damp problem, it is normal physics, and the only thing that matters is how quickly the moisture leaves afterwards.

That makes the extractor fan the whole story. Run it during the shower and for fifteen to twenty minutes afterwards, with the door shut so it pulls air through rather than dumping humid air into the hallway. A fan that stops the moment you flick the light off is doing about half the job it could.

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

Should I leave the bathroom door open or closed after a shower?

Closed, with the extractor running. Opening it spreads the moisture through the rest of the house, where it will find the coldest wall in a bedroom and condense there instead — out of sight, which is worse.

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 Eurostat for the second half of 2025. Last reviewed 21 August 2026.