“Why bother with the kitchen? – the problem’s in the back bedroom!” - here’s why….

Let’s assume the kitchen, hall and bedroom are all the same size!

Boil a kettle and make 60 grammes water vapour – this gas creates a "vapour pressure" in the kitchen.

If you then  open the kitchen window for an hour (and keep the door shut) the water is forced outside by vapour pressure – eventually the kitchen and outside will be equal - both 'air dry'. Fine on a nice day but in winter the kitchen will be frozen!

If you open the inner kitchen door the water vapour is evened out between the kitchen and the hall - 30gms in each space. If the bedroom door is also open the vapour evens out further and 20 grammes of water ends up in each room.

At 4am, when the house is at its coldest, the invisible water vapour condenses in the coldest surfaces - usually the far corner in the bedroom  and mould starts growing or on cold windows where water starts streaming down.

So water migrates from the kitchen, through the hall, onto the bedroom wall or cold windows….that is why you must stop steam from leaving the kitchen.

Boyle's Law 1676

Simple version. Water molucules tend to space themselves evenly in the air - the more air the less water per volume of air and the lower the humidity. So the quickest, cheapest, way to lower humidity is to add air in a controlled way.

Science - Pressure exerted by a gas varies inversely with the volume (at the same temperature)….
Stated as a formula, Boyle's Law V1/V2=P2/P1 (at constant temperature)where V1 equals the original volume, V2 equals the new volume, P1 the original pressure, and P2 the new pressure.