Last time I put windows in the insulation layer I used right angle metal brackets to hold the window to the wall then EPS up to window. EPS sealed to the window edges then 20mm EPS over the window frame face. No box used.

Posted By: fostertomAssuming air-sealing to the wall, and to the window frame, are do-able, what about air-sealing the four corner-edges of the box?

Can you not arrange that the ply box does not form part of your air barrier? Mine is the interior plaster over the box, for example (with added mesh and fibres in the plaster as necessary). But if the box was part of the air barrier then I'd put airtightness tape along the corner joins.

Re-reading your question, I see you might be asking about sealing the outer corners of the box itself, not window-to-box.

We did this with a waterproof tape. Again, very simple to do and it reduced my concerns about end-grain ply absorbing water.

NB. we also painted the surface of the timber with a marine-grade sealant once the windows were installed, just to be really sure.

Posted By: djhCan you not arrange that the ply box does not form part of your air barrier? Mine is the interior plaster over the box

Hmmm, for newbuild yes, but in a messy retrofit, where inner plaster, 2 skins of blockwork, maybe-CWI between, outer render, in some places blown-in Warmcel, all come together to form the reveal face, I fancy a sheet of ply or whatever sealed airtight to the reveal-edges of the blockwork, to shut all that behind a 4-sided airtight box, so plaster + render + blown-in Warmcel between them can do a half-decent airtight in-to-out job, without leaking sideways at such reveals! Then 20 EPS inside the ply, then a visual-only 12 OSB lining (or combined as insulated plasterboard), as well as the cillboard.

Posted By: Doubting_ThomasWe did this with a waterproof tape ... reduced my concerns about end-grain ply absorbing water

Sounds like you applied the tape over the external faces of the ply, not into the internal angle?
The former must happen before fixing the box inside the masonry reveals - much scope for the corners to be pulled apart a little or distorted during fixing, disrupting the tape?
The latter allows the ply to be cut/trimmed to each other while fixing one at a time, the tape then bridging any gaps you're left with?

Alternatively, the abovementioned 20 EPS inside the box gives space for a 19x19 bead or quadrant glued and tiny-screwed/stapled airtight into the box corners, instead of tape, all hidden by the visual-only lining. Tho perhaps, thus hidden from UV and wind pressure, I'd give more credence to tape.

Posted By: fostertompresumably def not OSB3

why not?

Bit late to this party, but in my new build I used 12.5mm 'External' ply.

I did my research and ordered the best spec I could find, but once fitted the exposed edges started to delaminate almost immediately. I immediately treated with Sadolins (I should have done this before fitting). I had the big sheets ripped down to the right size at a local business.

I used some Soudal mastic to seal the boxes to the frames before fitting, then sealed again when the window plus boxes were fitted.

What I learnt, was that for my windows, I really needed a 'slot' to fit the plasterboard to the side of the window and not butt it up against the window.

So next time (!) I'd put a piece of Ply round the window to give me the slot then put the box on top of that (25mm in total allowed in sizing).

Marine ply (BS 1088) should do the trick. Although my understanding is that marine ply is 'just' durable hardwood veneers with a WBP glue whereas WBP plywood (should) use the same glue but with softwood veneers. WBP ply or what ever the new standard is shouldn’t delaminate as a result of glue failure. Rot or water damage to the veneers is another matter.

But as I said above I installed windows in the insulation layer using steel brackets without the complexities of a box.

Posted By: fostertominteresting - but not clear to visualise - could you do a rough diagram?

One of the last posts I did on my website https://www.borpin.co.uk/2012/11/20/wall-complete-and-windows-going-in/

What I would have done is put an extra bit of ply, around the frame first to create the 'slot' for the Plasterboard.

The box might have been 18mm ply - it was 14 years ago!

Posted By: Peter_in_HungaryMarine ply (BS 1088)

I think I found that doesn't exist. They use EN specs now and, as I said, I got the best I could but it was still rubbish!

However, I am still really pleased with the result - it worked perfectly.