I am now well underway with my house renovation after a prolonged break due to illness, following on from my previous discussion.

We have settled on 200mm EPS EWI and are now looking at the windows. The wife is concerned that the sunken-window look won’t look very nice. The windows would originally have sat on the blockwork, approximately 200mm inboard of the finished face of the EWI.

I need to move them outwards, ideally in the easiest and cheapest way possible, as I’m having to make some sacrifices due to the costs spiralling.

Compacfoam is expensive, and cutting and setting out individual blocks would also be quite time-consuming for the builder. He has suggested simply using 100mm treated timber, similar to a fence post, and fixing it back into the blockwork, perhaps with some steel brackets along the bottom and possibly the sides.

I would also like to be able to remove the windows from the inside if anything goes wrong in the future, without having to destroy or significantly disturb the EWI.

Can anyone give me any guidance on whether the timber approach would be suitable, or suggest a better/cheaper way of achieving this? Windows approx 3G 1600mmx1700mm, doors 3000mmx2100mm and 2100mmx2100mm.

Secondly, I’ve also been quoted approximately £16/m² for graphite EPS beads to fill the existing cavity. Does that sound about right?

What sort of thickness plywood did you use? I made the mistake of specifying the windows as 1600mm wide when I should have made the opening slightly wider to accommodate the additional stuff, such as a plywood box. I think this was why I was thinking about a wooden frame screwed back into the blockwork like treated c16.

I have both approaches - 'windows-in-boxes' and existing windows EWI'd afterwards. I am also rubbish at taking proper illustrative pics. I can tell you that the deeper-set windows do not look rubbish (my deepest - renewed before we finally decided on EWI, have a reveal of nearly 250mm), but they are undoubtedly worse from a thermal bridging point of view. I think 21 Degrees (formerly Green Building Store) have or had a video re where best to place windows from a thermal point of view. I cannot remember where I saw it. It might be worth ringing them. Edit: Cannot find a video, but see this: https://www.21degrees.com/guides/installing-windows-in-energy-efficient-retrofits/

Thanks everyone, I think you have all given me a sanity check on how to proceed. Compacfoam - appeared expensive though I may check that again over the ply box option.

Posted By: englishjohnThe windows would originally have sat on the blockwork, approximately 200mm inboard of the finished face of the EWI.

I need to move them outwards

Yes, that's what you should - sat on the blockwork is a big thermal bridge - embed the windows within the insulation thickness, not touching the blockwork. Your choice of 200 EWI makes that a lot easier than e.g. Nick's 160. Within 215 (incl bedding, render etc), a 95dp window (like Russell Timbertech 1.1U 3G) leaves 120 for reveal depth plus a bit of space away from the masonry; 150EPS or 160 woodfibre leaves the reveal depth very minimal, and worse if your frames are more than 95dp (like Russell Timbertech PH grade 3G).

Posted By: Victorianecouse ... brackets?

Steel has conductivity 500x worse than ply (stainless steel half that), so even a few small-section steel brackets make a worse thermal bridge, in the inboard>outboard direction, than a continuous 'box' of 18 (I'd say 12) ply. Lately I've settled on the attached instead of either of those - track the heat-flow paths and I think it works better than the ply 'box' and can use Compriband instead of any sticky tapes.

Plan detail - extg cavity wall shaded on right showing extg window jamb with extg window removed. Going leftward from that jamb, a v short length of new stud wall - a 145x50 and a 195x50 studs, externally sheathed with 11 OSB3, 150 eps outboard of that, rendered. New window opening, new window fixed to 35x35 fixed to the 195x50. Note how, apart from the 35x35, the window is well embedded in insulation, minimum bridging.

Yes, the detail is the flank of a new bay window extension.

It refers to

Posted By: fostertomeven a few small-section steel brackets make a worse thermal bridge, in the inboard>outboard direction, than a continuous 'box' of 18 (I'd say 12) ply

and then comments

Posted By: fostertomLately I've settled on the attached instead of either of those - track the heat-flow paths and I think it works better than the ply 'box' and can use Compriband instead of any sticky tapes.

so yes it's incidental to

Posted By: djhmoving windows outboard to be in EWI instead of masonry

Posted By: Victorianecothey would be covered by the EWI also

But barely - both close to both outside and inside air (as is the ply box idea) thus v little insulation in the heat flow path.

Posted By: Victorianecoonly small in area compared to a timber box

600x smaller? No.

Frontal area, looking straight inward from outside:
Brackets say 3x25 in cross section @400c/cs = 187.5mm2 per m run.
12 ply continuous = 12,000mm2 per m run
600 x 187.5 = 112,500 vs 12,000 = 9.6x worse.

Posted By: VictorianecoI'm not sure? Surely a wooden box is more than 600x the width of a few fixings and a piece of angle bracket?

Do the sums for yourself, and become a believer

Well are you talking in pure u values? Or just surface area? Or a full window build up in either method?

Or are we factoring the brackets or fixings would be covered by EWI anyway? So it's not as clear cut.

I think we'd need to look at a whole window perhaps using the window industry standard sizing and compare the full build up. I think it would have negligible effect on the overall value of a window.

You still need fixings

Tom, how would you calculate the approaches?

Issues I foresee:

- The plywood box displaces the equivalent space of say EPS so this needs to be accounted for
- Angle brackets are hidden 'behind' the EWI so calculations need to factor full build up not just 'box v brackets'

Anything else one can think of to consider?

I did boxes, ply cut down from the sheets by a local firm. Tried expensive 'marine' ply - waste of money. I think I'd use OSB next time.

Used some sealant between frames and ply and sealed ply when installed.

Treat the outside edges of the ply/ OSB before installing (sadolins extra for example).

Posted By: VictorianecoSo assuming a masonry cavity wall. What is the box detail? 11mm frame screwed to inside of existing reveals? And then the windows screwed into/through that? Is that substantial enough?

Yes, although screw the box to the window first (easier to seal). A box has a high 'tuning moment' so will support a fair bit of window weight hanging off it. Don't forget insulated cavity closers if no full fill.

Where, in relation to the masonry cavity, will the window sit? Flush with the outside of the external leaf? Or the inside of the external leaf?

If doing it 'properly' i.e. incl airtightness, seal the box to the masonry, specifically to the render which can be airtight if not all cracked, seal the corner-joints of the box, and seal the windows to the box.