How much water vapour does a load of washing actually add to a room?

A single load of washing dried indoors can release around 1.5 to 2 litres of water into the air as it dries, on top of whatever moisture cooking, showering and normal breathing already add. In an average bedroom with the door shut, that's often enough on its own to push relative humidity above the 60% threshold where mould starts to grow.

Does opening a window while drying washing actually help?

It helps, but only if there's a genuine through-draught, one window alone in a closed room does little. A window cracked open on one side of the house with another door or window open elsewhere, creating real airflow through the space, clears moisture far more effectively than a single window in an otherwise sealed room.

Drying Washing Indoors Without Feeding the Mould

Drying clothes on radiators or an airer in the spare room is one of the most common, and most overlooked, sources of indoor humidity in UK homes. It's not the washing itself that's the problem, it's where that moisture ends up once the clothes are dry.

Where the water actually goes

A load of washing holds roughly a litre and a half to two litres of water once it comes out of the machine. Every drop of that has to evaporate somewhere before the clothes are properly dry, and if you're drying indoors, "somewhere" means straight into the air of whichever room the airer is standing in. Shut the door to keep the heat in, and that moisture has nowhere else to go until you open it again.

That's often exactly what pushes a room over the 60% relative humidity mark where black mould starts to establish itself, particularly in a spare bedroom or box room that doesn't get much daily airflow anyway.

What actually helps

A genuine through-draught, not just one open window. Cracking a window in the drying room while a door or window is open elsewhere in the house creates real airflow that carries moisture out. One window alone in an otherwise sealed room does very little.
Closing the door to the rest of the house, not opening it. Counter-intuitively, if you can't get a through-draught, it's often better to keep the drying room's door shut and its window open, so the moisture vents outside rather than spreading into hallways and other bedrooms.
A dehumidifier running in the same room, not a different one. A portable unit placed in the room where the washing is actually drying will pull the moisture out as it's produced. Put it in a different room, and the drying room still spikes in humidity while the dehumidifier works on air that was never the problem.
Extractor use, if the room has one. Bathrooms with an extractor fan are often the least-bad place to dry a small load, provided the fan is actually switched on and left running, not just used for the few minutes of a shower.

What doesn't really help

Drying washing on radiators without any additional ventilation is close to the worst option, since the warm air rising off the radiator carries the moisture straight up and around the room rather than out of it. A tumble dryer without an external vent or a working condenser has the same effect, adding a large amount of water vapour into the room in a short space of time.

The bigger picture

Managing one load of washing well genuinely helps. But in a home that's already running warm and humid from cooking, showering and normal daily life, laundry drying is usually the thing that tips an already borderline room over the edge, not the sole cause on its own. A whole-house system that keeps humidity controlled continuously means one wet laundry day doesn't turn into three weeks of a musty spare room, because the moisture gets extracted as fast as it's produced, wherever it happens.

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