I have a problem with mould on a single skin wall in a bedroom.
It's a 1930's 3-bed semi with bay windows. Downstairs, in common with the rest of the house, the lower part of the bay is cavity-wall brickwork.
The problem is that upstairs, in the front bedroom, the lower part of the bay is actually a 4-panel single-skin affair. Limited investigation show that it's a wooden framework filled with a mixture of broken brick and mortar, pebbledash-rendered on the outside and plastered on the inside.
The inside surface of this bay gets quickly covered with large patches of black mould. In winter, the surface is visibly damp.
I initially sealed the outside with waterproofer (Thompson's Rainseal or similar) to ensure that no water was coming through from the outside and then covered the inside with a double layer of 2mm polystyrene sheeting to try prevent condensation.
Alas, in a few months, the PVA glue failed and the sheeting peeled away from the wall because it was being lifted off by efflorescence. And, of course, it is all now covered in mould again. :(
Is it possible to /ever/ treat a wall like this to prevent mould occurring or should I simply do what I suspect is the 'proper thing' and erect some kind of inner skin to turn it into a cavity wall?
If I do have to turn it into a cavity wall, what's the best material (and method) to use?