I had a large lounge extension built last year involving a large steel RSJ to support the external wall. I extended the central heating 10 mm pipes to two radiators each side. Each branch went along the RSJ, round the end and down the former exterior wall. I then had it plaster boarded using the adhesive dab method and sheets.
Everything was OK until I turned off the central heating. I noticed damp patches appearing down one side in the original external wall. I made holes to investigate and found condensation beads on the pipe. now there is a big damp patch and mould on the underside of the steel RSJ at one end. There is no room to put tube type pipe insulation behind the plasterboard. One side of the room is perfectly all right with the same situation re pipework. My question is would making holes in the plaster board and squirting in expanding polystyrene foam be adequate to stop condensation or is the cavity wall allowing entry of damp air causing the problem?
I would be grateful for any advice