About ten years ago I had some plastering work done with sand / cement / additive plaster about 1m high across the front of the room, and the rest of the room just skimmed over. There is a persistant damp area above the line of the new plaster going up to ceiling height and appearing to a lesser extent in the hallway by the front door on the other side of the wall. Paint has 'bubbled' and some of the plaster skim has blown away from the wall.
One of the walls affected is cavity which has been filled with loose insulation, but the other can only be half a brick thick as it is the internal hall / front room wall extending past the front door where there is an open porch about 1m deep.
I am fairly sure that the damp is condensation but no other part of the room is affected. There is a radiator under a bay window to the right of this corner but being in a bay might leave the affected walls cold. I have installed trickle vents over the double-glazed wooden sash windows and the room doesn't 'feel' damp.
If necessary I can get at the cavity wall by removing the timber which closes the cavity where it turns into the bay, and drag out the insulation and check for water ingress, although obviously I'd rather not! Shoul I re-plaster, or is there something I can treat the plaster with? Should I insulate the half-brick wall outside the front door, and if so how?
Any advice gratefully received
Peter