I'm converting a small barn (9m x3.5m) for ancillary use, kids playroom, gym, office etc. Mixture of 9" double skin brick, 10" sandstone and weatherboard. So not to lose internal space and to insulate rather than use battens and insulated plasterboard has anyone taken out a layer of brick to make a single skin wall then built a stud wall on the inside with EPS between studs and insulated plasrboard (with vapour barrier) over (the stone would have to be halved to 4.5 inches or so). Cavity would be 25mm. Need underpinning and walls repaired/rebuilt. 

What's supporting the roof?

 

If it's a 9" solid brick wall, then I would think the bond would make it pretty impossible to just remove the inner layer.

 

What about external wall insulation?

Can't do external as AONB Andy have to maintain appearance. At the moments the wall plates are supported by the brick/stone walls or dwarf wall/4x2 studs on the side that is weatherboarded. My proposal would be for a 4x2 stud wall attached to the inside of the single skin brick/stone with DPC in between, 25mm air gap then 75mm EPS, 32.5mm insulated plasterboard over. Wall plate straddles both. Don't think bond is a problem, the walls are in a pretty bad way and largely need rebuilding, they're only 5'/6' tall. 

If it's in such a bad state why not just knock down and rebuild keeping external looking the same.

RichS, yes I need to keep the external looking the same, but rebuild double skin or single with stud? I don't want to go thicker than 9"/10". My concern is how good a rainscreen single skin old brick or 4.5" sandstone. 

4" brick laid in a new mortar will be fine - if you are worried they have gone porous then something like Thompson Water Seal will work but I've never had an issue with reclaim bricks and some have been 1880's Victorian ones. 

Thanks Peter, that's reassuring. These will be of that era and I'll be using lime mortar.