We have a 3 bed semi which we bought years ago. The property had a loft conversion done in 1979 which as you can imagine does not stand up to today's building regulations in many ways. For the past five years this has been used for nothing but storage partly due to it being too cold. I would like to tidy it up enough to use as a habitable room. I do have the intention of ripping the whole thing down in around 5 years to convert properly as part of a double storey extension project. The loft is made up of stud walls on all sides (even the party wall). The insulation in wall and ceiling is 25mm white polystyrene slabs. My question is how good an insualting material are these polystyrene slabs. Do I need to pull off all of the plasterboard and re insulate with Celotex or rockwool? Any advice would be greatly appreciated.