Drying out a damp room properly
After a leak or a spell of bad condensation, the temptation is to redecorate as soon as the surface feels dry. That is usually weeks too early. A wall can feel dry to the hand while the plaster behind it, and the masonry behind that, still hold a great deal of water.
Surface dry is not dry
Water moves out of a wall from the inside outwards, so the face dries first and goes on drying long after it stops feeling damp. Paint or wallpaper applied over it seals the remaining moisture in, and it then pushes the new finish off, blisters it, or grows mould behind it.
A rough rule for solid walls is a month of drying for every 25mm of thickness, in reasonable conditions. That is longer than most people expect and the reason so many repairs fail twice.
Stop the water first
Nothing below matters until the source is fixed. A room cannot be dried while it is still being wetted, and a dehumidifier running against an active leak simply runs for ever.
If the cause is not obvious, the pattern tells you: a sharp-edged patch that worsens after rain or after a particular room is used is a leak, while general spread across cold surfaces in winter is condensation.
Ventilation and gentle heat, not blasting
Drying works by getting moist air out of the room and replacing it with drier air. Open windows where the outside air is drier than the inside, which in Britain is most of the year but not a cold damp still day.
Gentle background heat helps, because warm air carries more moisture. Strong localised heat does not: a heat gun or a radiator turned up against one patch dries the surface hard, traps water behind it, and can crack plaster.
A dehumidifier is genuinely useful in a closed room, provided the room is kept closed while it runs. Empty it and keep going well past the point where it seems to be collecting little.
Take off what has failed
Blown plaster will not recover. If it sounds hollow when tapped or moves under pressure, it has lost its bond and needs to come off back to sound material. Leaving it and skimming over the top produces a repair that fails within a year.
Saturated plasterboard, and any insulation that has got wet, should be removed rather than dried. Both hold water for a long time and both grow mould out of sight.
Test before you decorate
The cheap test is to tape a square of clear polythene tightly to the wall and leave it for a day or two. Condensation forming underneath means moisture is still coming out and the wall is not ready.
When it is finally dry, seal bare plaster with a mist coat before painting so it does not pull the moisture out of the new paint unevenly.
When to get help
Water that has reached a ceiling below, a floor structure, or anywhere near electrics stops being a decorating job. Soft floorboards, a sagging ceiling, or a persistent smell with no visible source all mean something is wet that cannot be seen, and that is worth having assessed before anything is covered up.