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?
May I suggest that you buy a hygrometer/temperature gauge and place it near the problem wall, this will show how humid the room is.
Note, humidity varies between day and night and time of day.
It is usually coldest around 7am and therefore the air is at its wettest.
Mid day its warm and dry etc;
Unfortunately, cold water and pipes attract water vapour, condensation forms and walls and floors get wet.
Many combined hygrometers and temperature gauges provide a dew point indicator that alerts you to the possibility of condensation.
One simple solution is to run the heating between 6 and 8am at low temperature to avoid the problem.
Another is to use a small dehumidifier.
You can also open a window to let the water vapour out as water vapour always moves towards the cold or an area of low pressure and it is usually colder outside.
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