We recently moved house and are in the process of decorating.
The main bedroom had wallpaper which we stripped off back to bare plaster, which looks in good condition.
We put down a couple coats of suitable plaster paint as an undercoat, then proceeded to paint the room.
Most of the room looks absolutely fine, but as the picture shows, along the top of the wall the paint keeps going darker then returning to the correct colour throughout the day.
It is very cold to touch, so I am guessing it's moisture or damp related. It is the "external" wall of the property and the loft insulation doesn't look great.
It's a fairly consistent patch of wall and it happens every day.
We are getting someone to check the roof but thought I'd ask here for any suggestions.
Thanks for any insight.