Hi VE, I'll come back with detailed answers a.s.a.p. Nick

VE, You say:

''I’ve decided to move the new window frames forward out of the cold masonry opening and set them entirely within the new EWI layer using a cantilevered sub-frame box. My plan is to fix the windows into this box, seal for airtightness, and then lap the 150mm EPS EWI over the window frame perimeter by ~30mm.''

** I'm slightly confused here. If you use the box to post the window out into the insulation layer you won't be lapping '150mm' EPS over. I assume you'll post it out of the masonry by about 50mm so that you have a 100mm reveal. Am I understanding you correctly?**

1. Box Material & Dimensions
Material: I am planning to rip 18mm OSB3 into strips to form the box. Is OSB3 suitable here for strength and airtightness, or is exterior-grade WBP Plywood heavily preferred in real-world moisture conditions?

** I used WBP ply. My first thought was OSB, but ply 'felt' 'better'.**

Depth: Assuming a window frame depth of ~80mm front-to-back, should I make the OSB box the full 200mm depth (spanning the EWI + window overlap zone flush to the outer render line) to give a solid substrate for reveal insulation, or stop the box just behind the window frame face?

** Ah! I think this shows we are slightly at odds in our methods. I did not return insulation into the box; rather, I fitted the window into the box (before fitting, which answers a later question early!) flush to the outer face of the box, and you then post the entire box/frame unit (say) 50mm out past the face of the brickwork so that, when you apply the 150 EWI you 'make' a 100mm reveal.**

2. Construction & Fixing Methodology
Workbench vs. In-Situ: Is it better to pre-build a rigid 4-sided OSB box on a workbench and offer it up to the opening as one piece, or can I install the boards piece-by-piece directly into the reveal (starting with the bottom sill board, then jambs, then header) and screw the corners together in situ?

**In our case, none of the above!! We built the boxes around the frames, ply screwed to the frames and into 'itself' via the end-'grain'. Air-tightness tape over all joints. The 3G timber windows were already ridiculously heavy, so the weight of the ply only added a minor additional hernia.**

Fixings to Masonry: Should I use concrete frame screws (e.g., 7.5mm Spax/Thunderbolts) driven at a 45° angle through the OSB straight into the masonry reveals, or am I better off using heavy-duty steel angle brackets (L-brackets) on the exterior face of the brick?

**We just screwed through the ply boxes into the outer skin of brickwork. Lots of screws.**

Which Leaf to Anchor To?: Should the cantilevered box fixings anchor strictly to the outer masonry leaf, keeping the cavity and inner leaf completely untouched/thermally broken?

**See above for what we did. In reality it's a bit like adding sugar to tea, 'to taste'. If you want belt and braces, span both skins and get more screws in.**

Frame-to-Masonry Bedding: Should I apply a continuous bead of high-tack structural PU adhesive foam (e.g., Illbruck PU010) between the back of the OSB boards and the brickwork before driving frame fixings home?

**You could; we didn't.**

Picture Frame / Batten Alternative: Some have suggested using 3x2 (75x50mm) timber battens screwed flat to the outer brick face to form a "picture frame" that the window sits against. Does an 18mm cantilevered OSB box beat this on thermal performance and ease of taping, or are battens easier on site?

**Dunno! It's a matter of taste I guess.**

3. Window Sizing & Expansion Gaps
Perimeter Gap: When ordering windows, how much tolerance should I deduct from the internal box dimensions? Should I leave a 10mm gap all around (20mm total on width/height) packed with shims/packers and backfilled with flexible low-expansion window foam (e.g., Illbruck FM330)?

** We left no gap between the box and the frame - as above, the box was hard-fixed to the frame. We used packers to leave about a 8-10mm gap.**

4. Weatherproofing During Phased Work
Exposed Box Protection: If the OSB boxes and windows go in several weeks before the EWI system is applied, how should I protect the OSB box from driving rain?
Can I coat the OSB box with a liquid DPM / brush-applied airtight membrane (e.g., Blowerproof / Soliterm), or will liquid membranes stop the wood from breathing and trap moisture inside the reveal?

**I think I'd give it a 'roof-ette' of something, plus a similar arrangement to the sides and bottom.**

5. Specification & EWI Depth Sanity Check
EWI Thickness: I am aiming for roughly 150mm Grey EPS (Graphite) EWI, which seems to be the sweet spot for cost vs. performance (aiming for near-Passivhaus wall U-values around 0.14–0.15 W/m²K on top of cavity bead fill). Is 150mm standard practice here, or does jumping to 200mm deliver diminishing returns relative to the extra reveal depth and cill costs?

** In my experience 150 is easier to 'wrangle'.

Don't forget that, even if you use Compriband or similar between the outside front of the frame and the EWI, you need some sort of 'tanking', particularly on the bottom rail and across the EWI, so that the 'tanking' forms a 'drip' under the cill. Leave it sticking out at 90 degrees to hte wall at first till the rendering is done, then turn it down the face of the render.
Pro Clima do a thick butyl tape - Extoseal Encors, I think.**

Window Spec vs. Detail: Is it worth spending big on top-tier German/Nordic triple glazing, or does a mid-range triple-glazed unit deliver 95% of the performance once set correctly inside a well-detailed EWI box envelope?

** It's very much 'to taste' - I like timber; some like uPVC for its 'no rot' qualities. Try to get something like 0.9-0.75W/m2K whole-unit U value.

Sorry, no useful pics I am afraid. I get too busy fitting to remember my camera.

I hope this helps a bit. Opinions vary, so it may at least spark a debate. Good luck with it all.

P.S: For others using different materials, rigid wood-fibre for example needs more protection during the 'half-done' phase.

Posted By: Victorianeco150mm Grey EPS (Graphite)

Not directly relevant to windows, but when we were sourcing a 300mm thickness of Graphite EPS for under our floor slab, the cheapest price per cubic metre worked out as 150mm thickness (so for our job we used 2 x 150mm layers)

I see lots of scope for misunderstanding what's meant by descriptions, from all sides! Drawings badly needed!

My gut feeling (and AI, which I would trust as far as I could throw it) suggests that cement-fibre board is too brittle to be used in a cantilever situation, so that arguably knocks out the bottom rail, and I'd need someone better at physics than me to comment on my feeling that even the verticals could also be 'loaded'. And while I take your point, James, I think if the EWI (or the window indeed) is on fire I'd be more worried than about direct transfer from one to t'other. OTOH as a general rule fireproof over flammable is a no-brainer.

My suggested buildup

Think about your cills too

Hi Nick , hope your well and not grafting too hard!
The cement board can handle it and its worth remembering any project doing this is likely to have thick slabs of rigid insulation, which though aren't structural do have a load carrying potential.
My technic may not be structural engineer passed but once its all fixed in it seems pretty rigid.
I use dometime use chesp plastic 3g windows . My nightmare scenario is a kitchen or internal fire melting the frame and hitting the unprotected eps and spreading behind the render.
All that said , having done it a good few times I do wonder if moving the windows out on a refurbished is really time and cost effective

Borpin said: ''Actually - I'd do this (sorry for the micky mouse drawing)

Add in a piece of 12mm ply so you get a notch to slide the Plasterboard into.

Also, a stop bead for the render and EDPM expanding tape to seal rather than a Silicon seal - works brilliantly.

I realise the scale doesn't work but the bead and tape would cover the edge of the ply ''


Sadly I cannot reproduce the dwg, but isn't that quite an unmitigated thermal bridge reading from the 'cement board' to 'plasterboard' tags? Why not notch the insulation and bring it down to sail over the window frame?

** I 'lost this' when I tried to post it first, and in the meantime P-in-H has suggested roughly what I did.**

Posted By: Nick ParsonsSadly I cannot reproduce the dwg, but isn't that quite an unmitigated thermal bridge reading from the 'cement board' to 'plasterboard' tags? Why not notch the insulation and bring it down to sail over the window frame?

The bridge is no worse than the wooden frames of the windows.

Posted By: borpin

Posted By: Peter_in_HungaryI would stop the cement board inline with the ply

Building control wanted the edge of the insulation (PIR) covered!

Did the BC define what they meant by 'covered' and what they would accept as the covering? The edge of my EPS EWI was covered by a 3mm layer of cement based adhesive in which there was an embedded glass mesh (standard EWI corner profile)