The load-bearing wall between my hallway and my integral double garage is of 100mm dense concrete block. It should have been 150mm Celcon but the apparently the builder didn't have any to hand. To insulate it ready for the sale in a year or two's time, I thought of fixing timber studs to the wall, filling the voids with polystyrene foam then cladding with 11mm OSB with white paint finish. What thickness of foam would be enough, or should I use something else? The other walls are insulated, inner leaf Celcon 130mm, the ceiling is of TACfire sheets nailed to timber joists. The garage, like most, is used as a store room not a vehicle store.

There is 100mm of Rockwool on top of the ceiling between the joists. I realised afterwards that it would have been better to support it under the floorboards, but short of completely filling the voids with a blown product or taking down the Tacfire to push the rockwool up with a shelf of some sort, it can't be done.

Looking at your comments on the ceiling, I could take down the Tacfire sheets, fit thin OSB to support the rockwool against the floorboards, then refit the Tacfire.