Green Building Forum - Boxes for mounting windows outboard in the EWI zone Sun, 19 Apr 2026 10:47:58 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309988#Comment_309988 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309988#Comment_309988 Tue, 06 Jan 2026 12:45:55 +0000 fostertom Thickness - 15, 12, or will 10 ply be adequate?
Material - WBP ply? - any concerns about poss long-term intersitial condensation, positioned as they are well outboard in the insulation/sandwich? Is fibre-cement or other mineral board used instead? I guess interstitial condensation is no more likely than on/in the the window frame itself, but with less drying potential?
Assuming air-sealing to the wall, and to the window frame, are do-able, what about air-sealing the four corner-edges of the box?]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309990#Comment_309990 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309990#Comment_309990 Tue, 06 Jan 2026 13:33:16 +0000 Peter_in_Hungary Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309991#Comment_309991 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309991#Comment_309991 Tue, 06 Jan 2026 14:03:04 +0000 fostertom I previously used metal brackets, but after calcing their extreme conductivity, in the in-to-out direction, even at 25mm wide x 1.5mm thk @ 600 centres, some kind of timber 'picture frame' seems far better, and provides a continuous surface to air-seal to. Even chunkier 'picture frame' of e.g. 75x38 on-edge screwed/sealed to masonry external wall face, if it's well buried within the insulation.]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309992#Comment_309992 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309992#Comment_309992 Tue, 06 Jan 2026 15:53:34 +0000 djh 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.]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309994#Comment_309994 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309994#Comment_309994 Wed, 07 Jan 2026 16:07:53 +0000 Doubting_Thomas Posted By: fostertomwhat about air-sealing the four corner-edges of the box?

I've linked to this resource before, but there's a method called 'rabbit ears' for taping into corners. This is a US site but the principle still applies:

https://475.supply/blogs/design-construction-resources/zero-reveal-taping

Nothing revolutionary in what is proposed, and whilst it's a bit more fiddly that a straight edge, it's still something that any decent installer should have done before - pretty much all windows have corners!]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309995#Comment_309995 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309995#Comment_309995 Wed, 07 Jan 2026 16:11:11 +0000 Doubting_Thomas
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.]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309996#Comment_309996 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309996#Comment_309996 Wed, 07 Jan 2026 16:43:15 +0000 Peter_in_Hungary Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309997#Comment_309997 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309997#Comment_309997 Wed, 07 Jan 2026 17:27:01 +0000 fostertom 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 boxHmmm, 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.]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309999#Comment_309999 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=309999#Comment_309999 Wed, 07 Jan 2026 17:35:25 +0000 fostertom Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310001#Comment_310001 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310001#Comment_310001 Wed, 07 Jan 2026 17:44:35 +0000 djh Posted By: fostertompresumably def not OSB3why not?]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310003#Comment_310003 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310003#Comment_310003 Thu, 08 Jan 2026 12:37:54 +0000 owlman European classification is by EN314 and EN636. As I understand it if you use EU stuff in this application would need to be EN314-2, class3 for bonding type, and EN636-3 - class3 for durability. That would be the highest rating,but you may get away with class2 in each case for a slightly lesser protection

The WBP term may therefore be an indication of it's origin.
I'm not familiar with EWI requirements but I'm guessing you could use "Buffalo Board", phenolic resin coated birch ply or other similar product readily available, edges will still need sealing as with all other ply's.
Personally I'd avoid any particle boards, that's just personal preference.]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310273#Comment_310273 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310273#Comment_310273 Mon, 23 Feb 2026 07:36:39 +0000 borpin
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).]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310274#Comment_310274 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310274#Comment_310274 Mon, 23 Feb 2026 08:37:26 +0000 fostertom
Shocking about the delaminating. 'WBP' (water and boil proof) ply should withstand that - tho as a standard 'WBP' doesn't apply to the ply as a whole, but to the glue in it.]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310277#Comment_310277 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310277#Comment_310277 Mon, 23 Feb 2026 09:21:58 +0000 Peter_in_Hungary
But as I said above I installed windows in the insulation layer using steel brackets without the complexities of a box.]]> Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310281#Comment_310281 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310281#Comment_310281 Mon, 23 Feb 2026 10:18:48 +0000 fostertom Boxes for mounting windows outboard in the EWI zone https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310303#Comment_310303 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18391&Focus=310303#Comment_310303 Wed, 25 Feb 2026 16:18:04 +0000 borpin 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.]]>