I have always disliked the little dark, dank 3rd bedroom on the corner of the house at the back. Always disliked that room in every house we have had! Always gets damp, and it's always cold. Seems to develop mould patches on the ceiling and 2 outside walls too.
Got all the damp and mould off last Spring, and painted it with mould-resistant paint, (on the 2 outside walls and the ceiling,) and have a dehumidifier in there. SO FAR SO GOOD!
Hope it continues to do the trick!