Drying washing indoors without wrecking the house
Most advice about this is either wrong or unhelpfully vague. Here is what actually happens, with numbers from the same model the app uses.
Plenty of people have no choice about drying indoors — no garden, no outside space, or no money to run a dryer. This is not a lecture about whether to do it. It is about how to do it without ending up with damp walls.
Where the water goes
A full 7 kg load holds two to three litres of water when it comes out of the machine. Drying it indoors does not make that water disappear. It moves it, from the clothes into the air in your house, and it stays there until something takes it out.
That is roughly nine pints a week for a household doing four loads. It has to go somewhere, and if it is not ventilation then it is your coldest wall.
What a sealed room actually does
The model tracks the room as well as the washing, because the two are the same problem: as the clothes dry, the air gets wetter, and wetter air dries the clothes more slowly. In a room with no ventilation that feedback wins.
| Ventilation | Time to dry | Peak humidity | Hours above the mould threshold |
|---|---|---|---|
| Sealed | Never | 100% | All of them |
| Doors shut, 0.5 ACH | Never | 80% | 26 |
| Door open, 1 ACH | 29 hours | 71% | 1 |
| Window ajar, 3 ACH | 22 hours | 57% | 0 |
The first row is the one worth sitting with. In a genuinely sealed room the washing never finishes. It reaches an equilibrium with air it has itself saturated and stops, and everything after that is just moisture looking for somewhere cold to land.
The window question, properly answered
Every council leaflet says open a window. They are right, nearly always — but not for the reason usually given, and not always.
Ventilation only removes moisture when the air outside holds less water than the air inside. That is about dew point, not the humidity percentage on your weather app. Cold air at 90% humidity holds far less water than a warm room at 55%, which is why the advice works all winter even when it feels damp out.
On a mild, muggy, still day it stops working. Outside and inside hold about the same amount of water, so opening the window moves nothing — it just makes the room colder, which slows the drying down and makes things worse.
This is the one thing Pegday can tell you that a leaflet cannot: it knows both dew points, so it can say today whether the window will help or just chill the room.
What to do instead, when the window will not help
- Heat and moving air beat ventilation on those days. A fan on the airer does more than an open window.
- One room, door shut, with the extractor on if you have one, rather than spread through the house.
- A dehumidifier is the honest answer if you dry indoors often. It removes the water rather than relocating it, and it typically costs less to run than a tumble dryer.
- Spread it out. Crowding an airer is worth more than people expect — the model puts roomy against packed at about 25% on drying time.
Why it smells musty
Washing that takes more than about a day to dry starts to smell, and it is not the washing that failed — it is the drying. Bacteria get going on damp fabric well before it is dry, and once they have, rewashing is the only fix.
So the musty smell is a drying-speed problem, and everything above that gets the load dry faster also fixes it.
Condensation on the windows
That is the same water, arriving at the coldest surface in the room. It is not a sign the windows are bad; it is a sign the air is carrying more moisture than the room can hold at that temperature. Wipe it, and then deal with where it came from.
Figures from Pegday's own model — see how it works. It is a model, not a measurement of your house, and it is only as good as the room you describe to it.
Weather from Open-Meteo, licensed CC BY 4.0.