Help with mould/damp in room issue

[Boindsytin]

My duaghters room has lots of mold and damp. I cleaned it all around 6 months ago. Bought a dehumidifier and a small heater but admittedly after I washed it all off I was unsure what exactly I needed to do. Can someone talk me through it?
theres no furniture or anything against it but it is on the walls that are outside walls if you get what i mean.
should I be leaving window open?
leaving small heater on to dry out wall?
dehumidifier?
is there anything i can do to make sure this doesn’t come back.
please explain it to me in the simplest forms

Ventilation is probably key. Maybe crank the radiators up to max and open the windows and then leave the dehumidifer on all night. You can also buy anti damp paint that's like an undercoat and that also helps if it's condensing on the outside walls. We had a bad problem on an outside wall and had to redo the damp paint ever couple of years. If you're in a Victorian house though it's just something you have to live with as they weren't built to deal with central heating.

It's worth finding out the cause though. In a Victorian house it will probably be condensation. But if it's a more modern property and it's on an outside wall it could be rain or a downpipe coming in somewhere through the bricks. Is it low down on the wall or higher up? It might be worth getting a damp expert in to diagnose it and then seeing what you can do.

As pp says, ventilation is critical. Around the furniture and also ventilation with the outside. Sometimes it's insufficient to open a window - you need to also open something else in the house that will allow a good draught to really rush through (even if this is only for 5 mins per day). eg a window in another room or a door or whatever works in your house.

My ds is in a uni room with mould. I bought him a Meaco dehumidifier and he runs it 24/7. The humidity is always constant now, as controlled by the machine. You'd be astonished how many litres of water come out of it every week.

The walls need cleaning with white vinegar - not only to remove, but to kill the mould. Once they are clean and dry and you have ventilation and dehumidification going on, the mould should not come back.

Is the outside wall a cavity wall? I find the outside walls even if a cavity wall get cold during the winter, and condensation happens. I have to keep on top of it my opening the window and getting a draught through everyday. If the mould has really taken hold, try HG mould spray, but be very careful with it, it's incredibly strong.