I've had a window replaced with a smaller one and the gaps to the side bricked up. To save money I told the builder I would finish off the inside so it has been left as in the photo with just the outside skin. Rather than bricks/blocks on the inside, I was thinking of insulating the outside skin with 50mm cellotex, then building a timber frame to line up with the existing inside wall, finishing with foil backed plasterboard. I've read up on how to insulate a single skin wall, which is in effect what I would be doing, but can't find any consistent advice. Can I stick the cellotex straight to the outer wall, should it be battened, with or without an air gap? Should there be a gap between the cellotex and the plasterboard or should I aim to fill the whole space? And do I need to put any kind of damp proofing on the outside skin first? Any help gratefully received