Closing cavity after old sash removal and new window installed

[lewis.baker]

Hey all, looking for a bit of advice on how best to go about closing off the cavity around some new windows I've just had installed, the previous double glazing was installed within the sash boxes and this time round they've ripped them out and the windows installed in the external part of the wall, leaving the cavity to be closed off, however there's a pretty significant difference between where the internal and external walls end, about 100mm.

I've put a pic showing what I'm talking about.



I'm sure the proper method here is to brick up the difference but I'm a bit naff at brick work so looking to avoid that route. I've got some kingspan insulation boards and was thinking about making up the difference with that and then plasterboarding over it, not sure if that's a completely daft idea or actually a workable alternative, would love any input on it.

Cheers.

 

[^woody^]

2x 4x2 tanalised timber fixed to the internal leaf.
DPC up the inside of the outer leaf
Expanding foam to seal the cavity. Or your Kingspan and expanding foam.
DPC up the room-side faces of the timber to act as a vapour-check. Or polythene.
Plasterboard and skim.

 

[mrrusty]

I used thermalite blocks recently - easy to cut with a hand saw and a bag of mortar. DPC up the inside of the outer leaf. PB over.