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    • CommentAuthorgravelld
    • CommentTimeJul 13th 2017
     
    Check out this project in Ireland: https://youtu.be/uJyGE_kuEAA?t=193

    Here, instead of a plywood box, they mount the window on an external ledge of something called CompacFoam and then there are metal straps fastened to the reveals which hold the window, now outside of the old opening on top of the ledge, in place.

    Then, EPS is installed up to and over the window frame.

    Not sure if I really understand what all the white tape is doing on top of the CompacFoam...

    Anyone else do this and can comment on advantages/disadvantages over plywood box method?
    •  
      CommentAuthordjh
    • CommentTimeJul 13th 2017
     
    There's a brief description of the project in the video at
    https://mosart.ie/wp-content/uploads/2017/02/20PHT_AG15_McCormack.pdf

    The white tape is the watertightness layer, as it is labelled in the video, making sure that water falling on the window not only doesn't find its way inside the house but also doesn't find its way behind or into the insulation. It catches whatever isn't caught by the sill.

    I don't know of pros and cons versus a box; I supect it may well depend on whichever method the builder is most familiar with.
    • CommentAuthorgravelld
    • CommentTimeJul 13th 2017
     
    Watertightness - sounds like something new to learn about...

    So is it only applied to the top of the CompacFoam to stop water sitting on it, then penetrating into the material?

    I think the CompacFoam is encased by the EWI, so if rain got to it it does imply the weather penetrated the external insulation layer and therefore rain barrier doesn't it?

    Thanks for the link, that also explains the strange extra 50mm roof insulation I didn't understand from first view.
    • CommentAuthorSilky
    • CommentTimeJul 13th 2017
     
    •  
      CommentAuthordjh
    • CommentTimeJul 13th 2017
     
    Posted By: gravelldSo is it only applied to the top of the CompacFoam

    No, it is applied underneath windows.

    I think the CompacFoam is encased by the EWI, so if rain got to it it does imply the weather penetrated the external insulation layer and therefore rain barrier doesn't it?

    No it implies rain fell down the window and past the sill and behind the insulation.
    • CommentAuthorgravelld
    • CommentTimeJul 14th 2017 edited
     
    Posted By: Silkysimilar video here

    https://www.youtube.com/watch?v=GUUrd7dbBzU
    Thanks for that; more detailed.

    And thanks djh.
    •  
      CommentAuthordjh
    • CommentTimeJul 14th 2017
     
    Posted By: Silkysimilar video here
    https://www.youtube.com/watch?v=GUUrd7dbBzU

    That video has a much more complicated procedure. I'm not sure what is going on there and I certainly wouldn't want to have to do all that faff when there are simpler ways.
    • CommentAuthorborpin
    • CommentTime4 days ago
     
    I looked at CompacFoam and decided it was not cost effective. I also needed something strong enough to hang the windows on - the ply box is the only thing screwed into the frame holding the windows in place. Architect was sceptical but once he saw what it looked like he was more than happy with it.
    •  
      CommentAuthordjh
    • CommentTime4 days ago edited
     
    Posted By: borpinI also needed something strong enough to hang the windows on - the ply box is the only thing screwed into the frame holding the windows in place. Architect was sceptical but once he saw what it looked like he was more than happy with it.

    It's difficult for people to see where the strength and stiffness comes from, unless they've already thought it through. The sides provide the strength to carry the load, the bottom of the window essentially supports itself, with the box just closing the gap. There's no way an 18 mm sheet is stiff enough to support the centre of the window, for example. Although having written that, I remember that my entire house is cantilevered on 25 mm ply. :shocked:
    • CommentAuthorgravelld
    • CommentTime4 days ago
     
    I think I can visualise the sides providing the strength, but can you develop "the box closing the gap"? I thought by box you would mean the sides (and top and bottom).
    •  
      CommentAuthordjh
    • CommentTime4 days ago
     
    If the bottom of the box wasn't there, there'd be a big hole!
    • CommentAuthorborpin
    • CommentTime3 days ago
     
    Posted By: djhThere's no way an 18 mm sheet is stiff enough to support the centre of the window, for example.
    Not sure what you mean, but the 'box' means the window's (1200x1200 wooden frame 3G) inner edge is pretty much in line with the outer edge of the building frame it is attached to. No hint of movement after nearly 5 years. Top and bottom of the box play a significant role in opposing the cantilever effect.
    •  
      CommentAuthorfostertom
    • CommentTime3 days ago edited
     
    In your case maybe - but with thick say 250mm EPS EWI, the inner face of the window frame might be 90mm outboard of the stud frame. At 1200w, 18 ply would prob adequately support the frame cill nevertheless - but prob not if wider, and certainly not if it was 11 OSB (or non-flamable CPB) instead of 18.

    11 OSB is structurally adequate (and less conductive) for the vertical sides (carrying the weight in shear) and top (just a closer) of the 'box frame', whatever the window's width/outset - it's the middle of the bottom (cill) that needs support.
    • CommentAuthorgoodevans
    • CommentTime3 days ago
     
    My thoughts are..

    The extended brackets seen on the video will serve well to prevent doors/windows being sucked/pushed by the wind but are not sufficient to take the dead load.

    for wider windows more than two packing wedges are required under the window - so a box construction won't help for the intermediate supports - therefore I would suggest that the dead load of a window is best supported by a 12/13mm cement board shelf (it is as stiff as 5mm of steel) - Ply would work just as well - but cement board is not so readily degraded in case of water ingress.

    For the rest of the 'box' either 6mm cement board - or simply membrane will be sufficient (the extended braces work here). I'm not a window fitter but if window frames need 'squaring up' during installation some additional lateral pads may be needed at the top of the window to allow the window to be forced square.

    It is worth considering that a tall window or door will be able to overturn the blockwork under the cill (as the window is cantilevered out - depends on the overhang, weight of the blocks and the window weight) - so I think I would place some builders straps to the inside of the blockwork under each intermediate packer.
      window A.jpg
    •  
      CommentAuthordjh
    • CommentTime3 days ago
     
    Interesting to see a different point of view in different circumstances. I look at it this way:

    Think of sheet materials as being rigid in their plane and floppy out of it. Think of linear materials as being floppy in all dimensions except stretch along their axis.

    Then look at a window extended outwards in a box inside a timber frame containment. The only vertical plane objects are the window and the two sides of the box. So the window itself is stiff and supports itself. Its weight is taken by the two sides of the box and reacted onto the timber uprights. The box sides stiffen the timber against deformation in their plane. So the weight of the window is resolved but we're left with a couple because the window is outboard of its support. The couple is countered by another couple formed by the top and the bottom of the box (to any extent it isn't countered by the rigidity of the sides). So the top of the box is in tension whilst the bottom of the box is in compression. In both cases the box stiffens the timber members at the lintel and sill. The box also stiffens the window out of its plane to resist wind forces.

    So in my construction, I believe the bottom of the box is pushing the wall inwards, not trying to topple it outwards.

    In my case, I'm happy with OSB as the box material, since the window is timber, the surfaces the box is fastened to are timber and the whole lot is surrounded by straw! Fortunately from a fire perspective there's lime wrapped around everything.

    Incidentally, if you think about side-opening windows, or doors, it's pretty obvious the weight is carried through the side, since that's where the hinges are.
    • CommentAuthorgoodevans
    • CommentTime3 days ago edited
     
    However most window installers first lay and support the window from the bottom - and the brackets around the window back to the structural wall are floppy except in tension and compression (wind load). I first thought that the box would work as a unit - but with wider windows (1.2m or so) the sides have little effect with the load at the bottom centre.

    I do take your point that side hung windows have the load transferred to the side of the frame (and top hung windows presumable have frames that transfer the load to the side and are not hung from the lintels) but even so windows come in all sorts of configurations - the 'box' may not work for wider windows.

    If the widow manufacture states that the window base only need supporting at the edges - no problem, the box works well as DJH describes - if in the other hand the window has mullions and needs support under each mullion then each mullion is too far from the uprights of the box - you must therefore have a shelf to take the load back to the wall otherwise the widow will sag in the middle.

    With a shelf designed to take the load alone - any width of door/window works in the same way (but with over design for narrow openings) - and window fitters will feel secure perching the window on it's shelf. The same is true for window boxes however there may not be enough support in the middle of wide windows and excessive thermal bridging around the top and sides.

    The advantage of a wood based box is that the fitters fix the window to the box with no fancy brackets and the box is fixed back to the wall. Fancy brackets would be made of steel - I wouldn't be surprised if 1 bracket has the equivalent thermal conductivity of a few hundred mm of OSB. I need to think again - perhaps only mid span support is required for wide traditional windows and the box is a good all round solution.
    • CommentAuthorborpin
    • CommentTime2 days ago
     
    Posted By: djhSo in my construction, I believe the bottom of the box is pushing the wall inwards, not trying to topple it outwards.
    I agree with this except that inn steady state there should be very little couple. Couple is usually increased when the window is opened and which way that is depends on inward (T&T) or outward. Either way it is transferred to the side.

    From experience, a 1200x1200 window on an 18mm Ply box shows absolutely no signs of movement after 5 years.

    Posted By: fostertomIn your case maybe - but with thick say 250mm EPS EWI, the inner face of the window frame might be 90mm outboard of the stud frame.
    Only if I wanted the outside of the window in line with the outer skin (which I do not, I have deep reveals so the outside plane of the frame is still inside the insulation envelope).
    • CommentAuthorborpin
    • CommentTime2 days ago
     
    The last post I got around to on my building blog. All is now finished. Outside there was a 50mm gap then blockwork. You can see the supalux which BCO required to seal the edge of the insulation. https://www.borpin.co.uk/2012/11/20/wall-complete-and-windows-going-in/
    • CommentAuthorgoodevans
    • CommentTime2 days ago
     
    borpin, I like the idea of fixing the 'box' to the frame first - If you could do it again would you use the supalux board as your boxing material - what prep needs to be done to board like this (or cement board) before it can be plastered or skimmed?
    •  
      CommentAuthorfostertom
    • CommentTime2 days ago edited
     
    Posted By: borpin
    Posted By: fostertom... but with thick say 250mm EPS EWI, the inner face of the window frame might be 90mm outboard of the stud frame.
    Only if I wanted the outside of the window in line with the outer skin
    You must have mighty deep (160 from inside to outside) window frames; I was thinking 60 external reveal, 100 frame, 90 outset, adds up to 250.
    •  
      CommentAuthordjh
    • CommentTime2 days ago edited
     
    Posted By: goodevansthe 'box' may not work for wider windows

    My widest window is just under 3 m wide. Several are just under 2 m wide. They are all mounted in OSB boxes as I described. No sign of any movement after two years and counting.

    To be fair, I don't have any [fixed] mullions. The largest window is a fixed pane. The others are fixed, or have flying mullions (i.e. a master and slave window with nothing in the middle when both are opened) and some narrower ones are single opening. All openers are tilt-and-turn and I agree with borpin that when they are open the couple will reverse and so will the forces on the mountings.

    edit: I've just remembered that two of my 2 m wide windows do in fact have a mullion. They have a single fixed pane and a single movable sash, divided into two equal halves.
    • CommentAuthorgoodevans
    • CommentTime1 day ago
     
    This has troubled me - I could see DJH's couples and I could see my wall overturning force - both are right - but not at the same time - hopefully the two images show how this can be so.

    The top image assumes a simple 'pin' connection between the window and the shelf/cill and a fixed connection between the shelf/wall. In this situation the wall will have to resist overturning.

    The bottom image assumes the shelf/cill is rigidly fixed to the window but 'pin' fixed to the top of the wall. in this situation the lower window pain will be in torsion (with support from jams and mullions - so almost certainly no problem).

    In practice both the wall connection and the window connection will be fixed to some degree so a blend of the two will occur (analysis is beyond me here as the arrangement is over constrained).
      window B.jpg
    • CommentAuthortony
    • CommentTime1 day ago
     
    If you were to use the side cheeks for strength, the torsional forces get resolved within the inner skin of the wall.
    • CommentAuthorgoodevans
    • CommentTime1 day ago edited
     
    Yep I agree - sides are easy to support - but I think I'm satisfied that even in the worst case of mid span tension support on the inside of the wall is only needed if the box is not rigidly fixed to the cill.

    Now for air barrier....

    below is an idea for fixing a bullet proof air barrier - it can be embeded in to the external render and/or the internal plaster - only one gap to worry about (where the 'tube' joins).

    For a new build I see the frame arriving, fixing the tube of membrane using double sided foam tape, fixing 12mm cement board to the cill, 6mm to the jams and head, all flush with the front of the frame and lifting the whole lot into the rough opening finally fixing the cement board to the wall.

    the membrane can be trimmed back for the jams and head depending on the extent of plaster and render into the reveals.
      window C.jpg
    • CommentAuthortony
    • CommentTime1 day ago
     
    I would only go half way across the frame with the board, you may wish to 3/4 of the way across. Leave room for a good bead of mastic.

    Joining the tube, I would 'welt' the join, roll and fold.
    •  
      CommentAuthordjh
    • CommentTime1 day ago
     
    The membrane under the sill looks like mine. But note that it needs to be of a type designed for external use. I wouldn't want anything like that above my window since it might drive water inwards. My airtight barrier above and to the sides of the window is sealed to the frame but only extends inwards, not outwards. It's constructed in the manner approved by the manufacturer, out of separate pieces along sides, top and bottom and joints in the corners. But that's partly because it transitions to the internal plaster as airtight layer inside the reveal a few inches from the window frame.
    • CommentAuthorgoodevans
    • CommentTime13 hours ago edited
     
    I'm thinking housewrap as a membrane and double sided acrylic tape.

    Fixing the membrane to the window frame and then fixing the cement board to that before lifting into the rough opening seems like a plan to me with the installers packing / wedging / filling between the cement board and the blockwork.

    Alternativly build a OSB box into the rough opening, attach membrane to the frame and lift into the the OSB box - fixing from the frame side or the outside. packing wedging and filling between the membrane and the OSB. (OSB here as I presume the frame screws will hold better in OSB than cement board). I just don't know with the sensitivity wrt Grenfell if a fire resistant board will be insisted on.

    For the head and possibly the jams it may be worth putting some flashing outside of the cement/OSB box so that any water ingress from above is shifted elsewhere. To help the finish I was thinking of using pvc frameseal beads such as these ... https://www.wemico.com/pvc-window-frameseal-beads
    •  
      CommentAuthordjh
    • CommentTime10 hours ago
     
    With regard to whether you attach the box to the frame or the window first, think about how you are going to actually insert the screws. Also consider whether you are going to insert the window from the inside or from the outside of the house. The screws don't really hold in the OSB/cement board; they hold in whatever they are screwed into: the window frame, the timber support or blockwork etc.

    The building regs for tall buildings are considerably different than those for normal dwellings, especially if one or two storey.

    You certainly need something to direct water outwards around windows.

    The wemico beads are extremely useful.
    • CommentAuthorgoodevans
    • CommentTime6 hours ago edited
     
    Thanks DJH - I had expected that as neither the plaster or the render would be in place the windows could be installed either from inside or outside or a combination of both - I suspect you question indicates a lack of knowledge by me.

    Posted By: djhThe screws don't really hold in the OSB/cement board; they hold in whatever they are screwed into


    This indicates that for both OSB and Cement board you would have the screw head on the outside of the box and screw into the frame (as opposed to having the screw head on the inside of the frame and screwing into the OSB) This I can understand and would make window frame replacement really disruptive later in the buildings life (not a factor for me but to future owners not good).

    So other options would be:
    a) substantial timber 'box',
    b) using the extended braces to allow fixing the window into the blockwork.
    c) sticking/foaming the window into the box - with or without a 'stop'.

    Have you got a link/details of how you did your windows?
    • CommentAuthorgoodevans
    • CommentTime6 hours ago edited
     
    Thinking about it further - fix the window by screwing into the frame from outside the box - future owners can use option b) when/if replacement is necessary
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