Insulate wood framed box bay

[andyc2000]

Hi

Morning all.

I'm trying to figure out the best approach to insulate an upstairs box bay (house is circa 1910).

Double glazing has been fitted, but nothing has been done to the wall below and it is very cold and since removing a bay window seat a bit draughty as well. Construction is (was) lathe and plaster inside and then tiles over wood on the outside, there is no water ingress from the outside as far as I can tell, it's all nice and dry in there.

I won't be removing the tiles from the outside, so all work needs to be done on the inside. I've found a couple of approaches and wanted to confirm?

1. Dritherm and plasterboard over, no vapour barrier?
2. Celotex with a vapor barrier between that and the new plaster board?
3. Something else?

Photo attached.

Cheers.

 

[noseall]

Full fill with foil sandwich Celotex (between and across), foil tape joints and foil back plasterboards for dessert.

20160107_085741 (3)

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[andyc2000]

Is it OK to leave a gap between the celotex and the outside facing (25mm)?

I can't really get a full thickness in there, due to the nails from the outside tiles - gap is 100mm, otherwise could I use dritherm all the way through and then a 25mm celotex board over the batons?