Hi everyone, been reading lots of chats but still confused, hoping you guys can help! There is damp on a bedroom wall covering a chimney breast. Damp is in patches from floor to ceiling, but only on that wall. Damp gets worse with rain, then slowly starts to dry. Had no problems the last few years since I bought the flat. It's fully sealed, the chimney does not continue into the loft, it must have been removed during a previous partial loft conversion. Loft is completely dry. It is possible that bricks are at the bottom of the chimney. Below the bedroom is the ground floor flat, no damp problems there. Chimney breast is on an external wall, pointing seems fine, roofers have not noticed any potential cracks or holes for water. External bricks are damp though suggesting penetrating damp. Gutters seem fine. I've had a few damp investigators visit who all suggest replastering with a membrane. In my mind this doesn't seem to get to the root cause of the problem tho. My questions are: 1. There is no ventilation of the chimney breast, the damp guys have said this isn't a problem, yet everything I read online suggests it does need ventilation? 2. If ventilation is required, how should I go about it. Try to open up the chimney in the loft and put in a couple of tile vents on the roof (I can't actually find the top of the chimney breast even after lifting the insulation, so may need to bring someone in to look)? Insert a vent (and air brick?) in the bedroom? Install an external air brick / vent? 3. Should I waterproof the external wall, or will this make the problem worse by trapping in more moisture (even though it prevents more water from entering)? 4. Should look at both options 2 & 3? 5. What steps should I take, it may be a process of elimination so I guess simplest options first in the hope it resolves the issue? Any thoughts and questions welcome, thanks in advance for your help! Photos attached.