I have a problem with damp patches appearing on a wall in an unused bedroom of my house. I first assumed it must be something to do with my roof but after having the pointing re-done and some loose tiles replaced, the problem is exactly the same. I've checked and I've now realised that it happens when my central heating has been on for a couple of hours. However, a thorough check in the loft has revealed no issues with either of my water tanks (which are quite a distance from where the problem is occuring).
There's a problem with the radiator in that particular room and it's giving off very little heat (not sure if this is relevant).
I then thought the problem must be condensation and down to lack of ventilation and there is a vent in the room (on another wall) which I have now opened but the problem continues.
I had my house rendered about 3 years ago and I'm wondering if it's anything to do with that? However, it looks faultless outide (no visible cracks).
The water patch in not immediately noticable so it's hard to say how long it's been there but there is no sign of damp or discoloration - when it dries, it looks perfect again.
Any advice would be much appreciated.