Filling / insulating an alcove behind garage door.

[AndyWotsit]

I'm looking for some advice here.
We have this alcove area behind what was originally a garage door, modified in the 70's I believe and until now we've just hung a thick curtain across the gap with a subsequent build-up of mould, as you can see. I'm looking to fill and insulate this gap to bring it flush with the rest of the wall to make that space usable.
The alcove is 2.2m wide by 1.9m high, with slightly variable depth but not under 16cm.
My thinking is to coat the bay with a damp sealant paint then glue 100mm thick foil-faced insulation board directly to the wall. Then a 50mm batten framework around the opening and across as necessary to attach 9.5mm plasterboard to the battens. A skim over with roll-on plaster, so 100 + 50 + 9.5 + 0.5 = 160mm; Job's a good'un! (I would extend the window area with wood to make a tunnel through and glaze the inside also.)

Now is someone going to tell me that is a catastrophic way to do it and what I should be doing.
It does occur to me that the 2" air-space that this would create between the battens might be better served on the exterior side rather than behind the plasterboard.

 

[23vc]

Don’t do the damp sealant paint thing. I’m struggling to see how you can really make that work keeping the old doors and window in place. It’d need some tricky detailing for the window areas. But a framed celotex layer with vapour barrier on inside and breather membrane on the outside, with an air gap between that and the doors, may possibly be doable

 

[Tigercubrider]

Why not simply take the doors out and brick it up?
You should be able to find similar stone to match the house.
Adding a largish window would make visual sense with the lintel, and save on stone/brick.

My area has loads of 3 bed semis with integrated garages that have been bricked up and have windows fitted.
I think they simply create a cavity wall with insulation.

Some of your damp is likely due the old doors not sealing ingress from rain