Shower Steam: Beating Bathroom Condensation

A ten-minute hot shower releases roughly 0.2–0.5 litres of water into the air as vapour. That water does not vanish when you towel off — it is still in the room, looking for the coldest surface it can find. The fight against bathroom mold is really a fight about where that water goes next.

Why the mirror fogs first

Fog on the mirror is dew point physics in miniature. Shower steam loads the air with vapour until it is close to saturation. Any surface colder than the dew point of that air — the mirror, the tiles, the window pane, the toilet cistern — forces the vapour touching it to condense back into liquid water.

The mirror fogs first simply because it is smooth, exposed, and cooler than the steamy air. The same film of water is forming on your walls; you just see it better on glass. Left to dry in place day after day, that repeated wetting is what feeds mold in the joints and corners.

The one rule: keep the steam in the bathroom

The worst move after a shower is opening the bathroom door “to let the steam out”. The steam does leave — straight into the hallway and the bedroom, where it meets cooler walls and condenses there instead. You have not removed the water; you have distributed it.

So: door shut, and give the vapour an exit that leads outside.

With a window: open it wide immediately after the shower, door closed, for roughly 5–10 minutes. This is shock ventilation, and the bathroom is its ideal use case: maximum humidity, small room, fast swap.
Without a window: run the extractor fan during the shower and for a good while after, door closed. A small gap under the door lets replacement air in without letting steam flood the flat.

Cold outside air is a sponge

Airing a steamy bathroom in winter feels wasteful, but it is when it works best. Cold outdoor air holds very little water in absolute terms — even at a gloomy 90% relative humidity, air near 5 °C carries only a few grams of water per cubic metre, while your post-shower bathroom air carries far more. Swap the two and the humidity plummets; the walls, which barely cooled in five minutes, reheat the fresh air quickly.

This is why “it’s raining, no point airing” is a myth. What matters is the absolute water content of the air, and in the cold season the outside almost always wins.

Small habits that finish the job

Ventilation removes the vapour; a few habits reduce how much liquid gets a chance to soak in:

Squeegee or wipe the shower walls — that is water that never has to evaporate into the room.
Hang towels spread out, ideally not in the bathroom you just dried.
Do not dry laundry in a bathroom that already struggles with condensation.
If the mirror is still fogged fifteen minutes after your shower, the room is telling you the airing was too short.

Let the app watch for you

The right moment to air out a bathroom depends on what the outside air actually holds — not its relative humidity, which lies. Open/Shut compares the real water content inside and out for your location and tells you when opening that window will dry your home, and when it would do the opposite.