Hi- I'm in the process of retro IWI insulating my 300yr old stone cottage. I've lined the interior walls with 6" of celotex and had planned on fixing plasterboard over the top and giving it a coat of lime plaster but it occured to me that a simpler way may be to take off the foil skin of the Celotex facing the room, scratch up the foam and plaster directly on to the insulation. The depth of the celotex and the crumbly stonework actually makes it quite difficult to fix to.

I've done a couple of test pieces on scraps to see how it adheres but ;
a) has anyone done this before of have and suggestions/ views on whether the plaster would adhere longterm?
and
b) does anyone know how much the performance of the celotex would be reduced by removing the inside foil?

Your help, as always gratefully received
Bob

I would be worried about the boards shrinking and then the plaster cracking.

I think you'll then end up with a potential vapour barrier i.e. the remaining foil face of the Celotex on the cold side of the insulation and so face possible interstitial condensation?

Saint said:

''I think you'll then end up with a potential vapour barrier i.e. the remaining foil face of the Celotex on the cold side of the insulation and so face possible interstitial condensation? ''

That always bothers me anyway, in that in the vast majority of installations there will be *some* leak in the VCL. Any vapour getting past there meets another VCL! It does seem to complicate the 'sandwich'.

If you have kids charging around I'm not so sure it would survive. On the other hand you could avoid a few trips to casualty using its shock absorbing properties. Bit like soft-play centres but with a crispy coating.

On the vapour barrier threat I don't really understand (as in genuinely ignorant) , if there's no air gap between the lime and the foam where does the condesation form? If behind the foam I don't think it's that different to plasterboarding.

Condensation of the vapour could occur, if unchecked by a VCL on the warm side of the insulation, at the dew point somewhere around the interface of the inner face of the existing structure and the outer face of the insulation. The existence of a VCL albeit just the foil facing near that very interface will stop the vapour permeating beyond that point and so there would be a real chance of interstitial condensation forming within the insulation itself.