Repairs and Adding insulation to a cavity bay window

[joe90]

I Have a couple of questions about dry lining a cavity wall in a bay window situation.
Briefly the cross section beneath the window frame is:
Externally - hanging wood shalings attached to battens which are fixed to the main 3" x 2" construction of the bay. There is no felt beneath the battens and no insulation of any description (1950s build I guess).
Internally- foil backed plasterboard and a skim coat.

1. I recently had to replace some of the inernal plasterboard in one corner following water ingress from the old wood window which had rotted away over time (now replaced by a new uPVC frame set.) I could not get foil backed p/b so just used normal Gyproc palsterboard and a skim coat. Was this a mistake?

2. I would like to insulate this area for obvious reasons but did not want to strip off all the P/B to get into all of the cavity. Can I add a thickness of stiff insulation such as Jablite 50mm between some 50mm wooden battens straight against the existing finished plaster level and then a new plasterboard or ply finish to wallpaper to? (I know this will add about 70mm or so to the thickness of the bay walls, but it is big enough to take it without looking odd.) Also should there be any air gap in the new layers?

[TheDoctor4]

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