Internal Insulation Question

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Established Member
30 Aug 2021
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I have a large loft room which is used as my office. The one end wall is a party wall and is insulated on the neighbours side, the side walls are internal to roof and are insulated, as is the roof. My question is about the gable end wall, which is just a brick solid wall, no cavity and is uninsulated, just painted brick internally. It gets cold in winter and being the coldest wall, sometimes gets condensation on it. I now wish to insulate it, but see conflicting advice. Do I put the vapour barrier against the wall, then insulation, then plasterboard or insulation against wall then vapour barrier?

Thanks in advance for your answers
You need to prevent any warm air from reaching the cold face of the wall as it is the warm air that will be carrying water vapour. PIR board like Celotex can act as the vapour barrier if tightly fitted and all seams / joints are taped with aluminum foil tape. So some good info can be got from
Thank you Roy. Yes, have considered Celotex- the current wall is just painted brick with the brickwork not being especially flat. Have you any suggestions to get it to fit tightly without voids? Or would a thin layer of roll type fibreglass insulation behind the Celotex to take out the undulations be okay?
Vapour barrier on the warm side. Ideally then ventilate the cold side, ie leave a 25 mm gap behind the insulation and put a couple of air bricks in the wall.

The vapour barrier is to prevent and moisture getting to the cold wall and condensing, if some does make it past/around then the ventilated cavity allows it to evaporate and be removed in warmer/dryer times. That’s the theory.


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