I've ended up doing it like these, depending on whether 100 or 150 EWI - tho in this case it's EWI applied to a newbuild timber frame.

I did the plywood box option:

24mm marine ply coated externally and endgrain sealed with CPES clear penetrating epoxy sealant (probably the most toxic thing I used on my project, but apparently used regularly on boat decking to get a good watertight finish). Taped back to the structure on all sides using a Pro Clima tape (Extoseal Encors).

The box was supported by 50mm battens in a 'picture frame' around the outside and ultimately connected back to our Larsen Truss cladding frame, but the windows were fine just supported in the plywood box for a while.

Posted By: Peter_in_HungaryIs there a generic drawing showing the details?

Have you found the Retrofit Pattern book? Not something I've used myself, but it has a sectional drawing and some interesting facts and figures:

https://retrofit.support/detail/73/

<blockquote><cite>Posted By: tony</cite>I fix windows to the outside of the outside skin, frame slightly larger than the opening!</blockquote>
This is what I was thinking to do.

And it would seem that normal softwood 'picture frame' fixed to the brickwork will be acceptable

I used marine ply. Echo Doubting Thomas' comment re sealing. Go 'overboard'! I thought we had it all done and dusted but still had some ingress issues - now predominantly sorted.

Posted By: Nick ParsonsI used marine ply.

What is generally available here is what is described as water and boil proof Birch ply (which I presume is the equivalent to the UK WPB ply)
Would there be any advantage to use this rather than normal building grade softwood

Posted By: wookeyFor a masonry wall I prefer structural insulation over a timber box. So I've done it like this:
http://www.wookware.org/house/retrofit/drawings/window-EWI-side-100.pdf
Alma vert is a cheaper option than compacfoam in the UK/Ireland now.

That looks like this: http://wookware.org/house/retrofit/pics/html/053-EWI-window-outboard.jpeg.html
(XPS used in the non-structural gaps there which is fiddlier but cheaper :-)

It's got better psi values than the wooden box. And you get 30mm more window for a given opening.

I looked at compacfoam here and the blurb describes it as 200 kg/m3 EPS which I presume would otherwise be known as EPS 200. or is there a difference?

one of the demo youtube clips shows a piece about 70 mm square and about 90 cm long having a bead of silicon adhesive applied to the back and then fixed to a wall with 3 window fixing screws, a further 3 pieces similarly fixed to make a box then a window fitted into the box (with a gap at the sides presumably for squirty foam)
With the box made the window was fitted with window fixing screws in to the compacfoam. The squirty foam was not shown.

Not withstanding the picture in the link given above does the compacfoam need the insulation under and around the box to avoid sagging in the long term? i.e. does it rely on additional support from what surrounds it?

Given that XPS here is also quoted as 200 grade would that perform the same or is there a substantial difference between compacfoam and XPS 200 in strength and structural performance?

Posted By: Victorianeco1. Do you really need airtightness tape?

Fundamental question for you. Do you believe that airtightness is worthwhile?

If not, there's no point asking your question. If so, then how you achieve airtightness is up to you - there are many different ways and airtightness tapes are just one. But equally general sealant is certainly not one of them.

Because sealant doesn't stay sealed for long.

edit to add: EPS isn't airtight. External render cracks.

Posted By: VictorianecoSo if you use EPS for EWI you're saying you also need to wrap it in an air tightness tape also? Hmmm

No, I'm saying you need to think about an airtightness layer separately from your insulation, whatever it is. Please read some guidebooks.