INSULATING A 100MM THICK WALL

[shanie]

hi im converting a shed attached to the house into a utility room. i want ed to construct a 50 x 50mm stud work and attached a thin layer of ply to the back of it. then insert 50mm celotex to fill up between the stubs. then finsih of with 12.5mm of foil back plasterboard. i was going to leave a small gap of 18mm behind the studwork so it wasnt touching the brick wall behind. do anyone think im going to get problems with condensation or anything else. any ideals with be gratefull.

[welsh brickie]

50x50 studs at 400mm centres, 50mm drytherm insulation inbetween,vapour barrier stapled to studs,then plasterboard