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Housing - Renovation: Installing new windows before EWI on cavity wall

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  1.  
    Has anyone installed new windows in a (filled)cavity wall using plywood box, then temporarily weatherproofed the outside, whilst waiting for the EWI to go on - then pushed them out further into EWI when thats installed. I am getting my windows next week, but will be some (winter) months before i can do the EWI. I just watched the carbon coop / GBS video about window installation and this was mentioned as possible but maybe difficult.

    thank you in advance.
    Kai
    • CommentAuthorSimonD
    • CommentTimeSep 25th 2022 edited
     
    I've done this for a whole winter. I did not pushed the windows out further into the EWI once installed, they were installed into the plywood box in the correct position for final fitment of the EWI.

    All I did was used exterior grade plywood, sealed up with linseed oil, sealed the plywood box to wall junction with exterior sealant. When I installed the windows, they were sealed up on the outside and with airtightness foam between window and plywood. I do have large roof overhangs but experienced no problems even throughout a wet and stormy winter.

    Just be mindful that in your design you leave sufficient reveal depth around the window as you're going to want window beads and corner beads around the window opening. If this isn't deep enough, you'll need to find an alternative method to reinforcing the ewi corner, assuming you're rendering.
  3.  
    Ive done it several times. I prefer a cement board box for fire spread reasons.
    • CommentAuthortony
    • CommentTimeSep 25th 2022
     
    I like windows against the outside skin 10 to 15mm larger than the opening all round. More daylight in less problems.
  5.  
    thank you all for the comments, will take on board.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 26th 2022
     
    Posted By: tonyI like windows against the outside skin 10 to 15mm larger than the opening all round
    Yes, I get the reveal plaster face to hit the window frame inner face 25mm (or a bit more) in from its perimeter, and the outer reveal face of the EWI in the same plane.

    I don't do ply (or cement board) box, but 35w x 70 or 95dp 'picture frame' long-screwed to the wall face over expanding foam (aka Compriband) strip. The inner size of the 'picture frame' is 40mm larger all round than the window frame's perimeter. 35x35s are glue/screwed inside the 'picture frame', so there's a 5mm gap all around the window frame perimeter. The frame is screw/shimmed inside the 35x35s, again over expanding foam strip.

    It's all very robust, incl taking the window frame's (or door frame's) weight under a long cill. No sticky tapes are used for airtightness. This mounting frame ends up more deeply buried in insulation (further from outside or inside air) than with the board box method, and the window frame's perimeter is buried 25mm (or a bit more) deep into the insulation.

    This should be readily weatherproofable over a winter before EWI is fitted.
    • CommentAuthortony
    • CommentTimeSep 26th 2022
     
    Love it Tom , I just don’t use any wood at all. Agree with wrapping the EWI over the frame, nice
  8.  
    Posted By: tonyLove it Tom , I just don’t use any wood at all. Agree with wrapping the EWI over the frame, nice

    So what supports the window ? Steel brackets ?
    • CommentAuthortony
    • CommentTimeSep 26th 2022
     
    In my case short steel dowels into the reveals and support in the middle at the bottom, angle bracket, yes
    • CommentAuthorbhommels
    • CommentTimeSep 26th 2022
     
    Posted By: tonyI like windows against the outside skin 10 to 15mm larger than the opening all round. More daylight in less problems.

    This advice is one of the GBF gems!
    For supporting the windows I used a long ledge beam (ex floor joist). I did not see the need for a full picture frame, and have used steel brackets for fixing the sides and head to the masonry.
    Due to the outer brickwork being a bit uneven, I did use airtightness tape which is also compressed in the window-masonry joint in addition to foam tape.
    If you go for windows opening to the inside (which I did), do make sure all the dimensions, inside and outside, are correct. I found it surprisingly tricky to measure the thickness of the parge coat and plaster layers on the inside relative to the outside reveal.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 26th 2022 edited
     
    Posted By: Peter_in_HungarySo what supports the window ? Steel brackets ?
    No, the window sits within the 'picture frame', specifically within the 35x35s glue/screwed around the inside of the 'picture frame', so there's a 5mm gap all around the window frame perimeter. The frame is screw/shimmed inside the 35x35s, over expanding foam strip.

    Posted By: tonyshort steel dowels into the reveals
    I can't picture this. The frame is wholely outboard of the wall, against its outer face but only contacting a 10-15mm band of wall. Do the dowels go into the masonry on the diagonal?

    Posted By: bhommelssteel brackets for fixing the sides and head to the masonry.
    Steel is 400x as conductive as wood. So say the steel brackets are 3x25mm (flimsy - some say 5x30 batstrap) in cross section @ 300c/cs, that's .00025m2 of steel per m run, conductively equiv to 0.1m2 of timber per m run, or 100mm wide continuous (looking at the wall face) vs 12mm thick board (looking at the wall face)! So steel brackets or any steel running in direction outside to inside are a massive cold bridge.
    • CommentAuthorborpin
    • CommentTimeSep 26th 2022 edited
     
    Posted By: fostertomNo, the window sits within the 'picture frame', specifically within the 35x35s glue/screwed around the inside of the 'picture frame', so there's a 5mm gap all around the window frame perimeter. The frame is screw/shimmed inside the 35x35s, over expanding foam strip.
    Still cannot 'picture (groan) this detail.

    I did the box method. Screwed (and sealed) the box to the window frame then screwed (and sealed) the box to the house frame so the window 'stuck out'. Outer edge of box aligned to outside of window. I got a local window firm to cut down the sheets of ply into strips (better finish than doing it on site).

    120 mm (IIRC) PIR on outside of frame such that window was now at same depth as insulation (insulation buts against box).

    Deep reveals in Scotland so the outer blockwork then came against that. Render - render stop strip on inside of reveal leaving a 10mm gap to the window frame filled later by expanding foam tape.

    My issue was that I should have put a second strip round the window frame itself first, set back from the inner face, and created a slot for the plaster board to go into rather than it butt against the window frame.

    Second issue was, I did not treat the 'external' Ply exposed edge so it started to delaminate. Caught it quick enough but would have treated the wood first if I'd realised this.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 26th 2022
     
    Ere y'are
      Picture frame window mounting.PNG
    • CommentAuthorbhommels
    • CommentTimeSep 27th 2022 edited
     
    Posted By: fostertom


    Posted By: bhommelssteel brackets for fixing the sides and head to the masonry.
    Steel is 400x as conductive as wood. So say the steel brackets are 3x25mm (flimsy - some say 5x30 batstrap) in cross section @ 300c/cs, that's .00025m2 of steel per m run, conductively equiv to 0.1m2 of timber per m run, or 100mm wide continuous (looking at the wall face) vs 12mm thick board (looking at the wall face)! So steel brackets or any steel running in direction outside to inside are a massive cold bridge.

    Wait - this is not how it is done. Since the windows are oversized, the steel angle brackets securing the window casing to the masonry at the frame sides and head (as well as the ledge beam BTW) are all buried under the EWI which is extended to lap the window frame, similar to your detail. No cold bridges here!
    •  
      CommentAuthorfostertom
    • CommentTimeSep 27th 2022 edited
     
    But the brackets are still a heat loss route (as are my timber 'picture frame') bypassing the EWI even if buried in it.

    Two factors at work -
    one, already mentioned is the cross sectional area x conductivity;
    the other is where the outer and inner ends of the brackets (or my 'picture frame') start and finish on the inboard-to-outboard temp gradient thro the wall sandwich. If the outer and/or inner ends start close to outdoor or indoor temp on the gradient, the brackets conduct a lot of heat. If the outer and/or inner ends are well buried in the EWI and only bridge part of the gradient, they conduct less heat.

    Metal brackets tend to start and finish very close to outdoor and/or indoor temp; my 'picture frame' has quite a lot of insulation outboard and inboard of it, on the shortest heat path thro the EWI and interior reveal insulation, probably bridges only one third of the gradient.

    The board 'box' approach also puts the board ends close to outdoor and/or indoor temp, and extend a long way in the out-to-in direction. For instance
    Posted By: borpinOuter edge of box aligned to outside of window
    Would be less of a thermal bridge if the box only lapped the window frame edge face by say 30mm, just enough to screw thro, not going all the way outboard..

    Note that even steel that lies transverse to the out-to-in heat-flow path has negligible effect as a thermal bridge.
    • CommentAuthorbhommels
    • CommentTimeSep 27th 2022
     
    Posted By: fostertomBut the brackets are still a heat loss route (as are my timber 'picture frame') bypassing the EWI even if buried in it.

    Well, yes, but, no, but.
    Since the windows are GBS, the frames are 70+mm wide so that is the EWI lap length. Not quite the full EWI thickness I admit but adequate in my opinion.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 27th 2022
     
    Posted By: bhommels70+mm wide
    Does that mean deep in-to-out, or wide on face? Pics on GBS site don't look like they can be 'buried' (mastered by the reveals) any deeper than 25mm.
    •  
      CommentAuthordjh
    • CommentTimeSep 27th 2022
     
    AIUI GBS windows are 78mm deep, Tom. The whole depth can be buried I believe - I can't think of a window for which that isn't true? And whilst I understand what you're saying about steel straps, as Bart says they don't go all the way through by any means. Also I don't believe they need to be at 300 mm c/c - a couple of brackets on each side and on the top should be enough, no?

    If they were that bad I'm sure people would sell stainless brackets :) And I expect our PH architect and the certifier would have objected to their use. Most of ours were installed in OSB boxes but there's the odd few using brackets (all doors IIRC).
    • CommentAuthortony
    • CommentTimeSep 27th 2022
     
    From tony, yes on a diagonal
    •  
      CommentAuthorfostertom
    • CommentTimeSep 27th 2022
     
    Thanks tony - tho you'd have had trouble with the cav wall jambs as drawn above (1954 council house) - common enough for one leaf's opening to be significantly wider than the other.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 27th 2022 edited
     
    Posted By: djhThe whole depth can be buried I believe
    Sure, like the 105mm depth of the one drawn above - but still only 'buried' 25mm in the 'width' plane - 70 vs 78 vs 105 makes little difference as implied in
    Posted By: bhommelsthe frames are 70+mm wide so that is the EWI lap length
    to insulate/spacing the bracket-end away from reveal outdoor cold source.

    I'd put flimsy 3x25 (or less) brackets @ 300c/cs averaged all round incl poss closer at corners and under cill; 5x30 batstraps wider.

    Typically, brackets go from outside of frame, very close to reveal outdoor cold source, right through to excellent thermal contact with wall masonry, which is near enough indoor temp. My timber ones go 2/3 that distance or less, insulate/spaced well away from the cold reveal, and contact points a bit isolated by Compriband foam, bridging I guess about half that delta-t on the gradient. I should do a Therm model but am pretty sure about it.
    •  
      CommentAuthordjh
    • CommentTimeSep 28th 2022
     
    Posted By: fostertomTypically, brackets go from outside of frame
    Why would you do that? A screw near the end of the bracket into the meat of the frame (maybe the centre at most) is the purpose of the bracket, so why extend it any further?

    The design we're talking about has solid underpinnings taking the weight and brackets around the top and sides just to restrain the window. I still say your 300 c/c is overkill.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 28th 2022 edited
     
    Posted By: djhA screw near the end of the bracket into the meat of the frame (maybe the centre at most) is the purpose of the bracket, so why extend it any further?
    Agreed, but e.g.
    Posted By: borpinOuter edge of box aligned to outside of window
    often applies to bracket method too. Even so, I've drawn these methods out on section often enough to see which ones a) bridge more of the delta-t on the heat gradient/path, and b) place an end closest to outdoor reveal temp and/or indoor temp. Sounds like a duel - Therms at dawn!
  24.  
    Posted By: djhA screw near the end of the bracket into the meat of the frame (maybe the centre at most) is the purpose of the bracket, so why extend it any further?

    When I put windows into the insulation layer I used metal brackets screwed to the wall and long enough to cover the window frame then fixed the window frame to the brackets from inside of the frame. The reason being that if in the future the window needed changing the screws could be seen and the window changed without too much destruction. And yes the EWI was extended over the window frame by 20mm. (inward opening windows)
    •  
      CommentAuthordjh
    • CommentTimeSep 28th 2022
     
    Posted By: Peter_in_HungaryWhen I put windows into the insulation layer I used metal brackets screwed to the wall and long enough to cover the window frame then fixed the window frame to the brackets from inside of the frame. The reason being that if in the future the window needed changing the screws could be seen and the window changed without too much destruction. And yes the EWI was extended over the window frame by 20mm. (inward opening windows)
    I'd be interested to see a drawing, or photo. The main reason for screwing the bracket into the side of the frame is primarily aesthetics - there's nowhere on the inward face of the frame that isn't visible when everything is finished, apart from the thickness of the plaster. A secondary reason is so the screws aren't loaded in tension and perhaps more likely to pull out. Our EWI is extended over the outer face by more like 50 mm.
    •  
      CommentAuthordjh
    • CommentTimeSep 28th 2022 edited
     
    Posted By: fostertomAgreed, but e.g.
    Posted By: borpinOuter edge of box aligned to outside of window
    often applies to bracket method too.
    Well that's the point isn't it? A timber box isn't as much of a thermal bridge and it forms a basic rainscreen (and airtightness if taped) so there's some justification for not finishing the box at the middle of the frame. But brackets don't have any of those excuses so there's no reason to draw or install them that way that I can see.
  27.  
    Posted By: djhThe main reason for screwing the bracket into the side of the frame is primarily aesthetics - there's nowhere on the inward face of the frame that isn't visible when everything is finished,

    I put the screws on the inside face of the frame not the internal side of the frame so when the window is closed the inside face is covered by the window. No screws to be seen unless the window is wide open.
    • CommentAuthorbhommels
    • CommentTimeSep 28th 2022
     
    Posted By: fostertom
    Posted By: bhommels70+mm wide
    Does that mean deep in-to-out, or wide on face? Pics on GBS site don't look like they can be 'buried' (mastered by the reveals) any deeper than 25mm.

    I'd love to post a drawing but I am struggling!
    •  
      CommentAuthordjh
    • CommentTimeSep 28th 2022
     
    Posted By: Peter_in_HungaryI put the screws on the inside face of the frame not the internal side of the frame so when the window is closed the inside face is covered by the window. No screws to be seen unless the window is wide open.
    Ah, your windows are evidently quite a different design to ours. A lot of the frame is still visible when the sash is closed, and of course on fixed windows there is no moveable sash.
  30.  
    Posted By: djhAh, your windows are evidently quite a different design to ours. A lot of the frame is still visible when the sash is closed, and of course on fixed windows there is no moveable sash.

    Yes - all casement windows

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