Hi everyone,

I’m finalising the detailing for replacing my windows as part of an External Wall Insulation (EWI) retrofit on an existing masonry cavity wall property, and I’d love to tap into the real-world experience of those who have executed this on site.

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.

Before I lock in the exact window unit dimensions and place the order (there is an 8-week lead time I want to get ticking!), I want to sense-check my proposed execution and get your thoughts on a few specific questions.

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?

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?

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?

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?

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?

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?

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?

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)?

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?

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?

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?

Any practical advice, site photos, or "things I wish I knew before doing this" would be hugely appreciated so I can get these opening dimensions finalized and get the windows ordered!

Thanks in advance!