Found some damp high up in the corner of our bedroom. It's a 1930 semi and the corner has a internal and external wall with the airing cupboard behind the internal wall. This area was investigated, when we removed wallpaper to redecorate we found a purple coating of paint covering about 6 inches from the corner both ways and 12 inches down from the ceiling. Using a damp meter to check the wall we found this corner to be damp, it did not show through onto wallpaper and plaster is solid. but we are having the walls and ceiling re plastered as planned. Investigating further to locate possible ingress of water found no defects or leaks but did find an old chimney behind this corner cut off at ceiling level after new roof and above kitchen ceiling after extension with both ends open (dropped debris down). Kitchen extension roof has two ventilation caps, and a good blast of air into loft through this chimney when outside kitchen door opened. Now not certain what to do, if anything to rectify this problem.
Thanks a lot for your answer, the outside wall around the window which also included the small damp area has had all plaster removed ready to have insulation boards fitted and then re-plastered. The rest of the room will only be re-skimmed apart from the ceiling which will be over boarded and finished. If it would be more beneficial i could remove a section of plaster from the adjacient wall at the top and expose the brickwork and work directly onto this with the block and DP paint, or would this cause a problem with the bonding of the plaster. Would a PVA coating be good enough to seal this corner? Again thanks for your help.
Without seeing the problem it is difficult to suggest a solution.
I think you should leave the internal plastered wall as is and apply damp-proofing paint around the corner before fitting plasterboard on the outside wall.