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    • CommentAuthorgravelld
    • CommentTimeJan 11th 2018
     
    • CommentAuthordelprado
    • CommentTimeJan 11th 2018
     
    I think thats agreeing with me isnt it Gravel?
    • CommentAuthorgravelld
    • CommentTimeJan 12th 2018 edited
     
    Don't think so. If possible I'd rather we continue the discussion on the validity of the idea in that thread.
    • CommentAuthorTimSmall
    • CommentTimeJan 12th 2018
     
    What's in the cavity at the moment?
    • CommentAuthorgravelld
    • CommentTimeJan 12th 2018
     
    At the moment? Mineral wool, partial fill. During the works I will (best case) find some sort of expanding foam CWI to insert with it and push it against the internal leaf or (worst case) pour the beads myself and seal the top of the cavity anyhow.
    • CommentAuthorgoodevans
    • CommentTimeJan 12th 2018 edited
     
    Posted By: gravelldTo make sure I understand: you are here installing the bottom of the box, placing and fastening the window on that, then sliding the jambs and head of the box in and fastening?


    If you want to - yes - this is what the '4 walls' guy did in the link in your original post.

    There are two main 'window box' methods - the '4 walls' version with chunky timber on the outside - or thinner ply/osb fixed to the rough opening reveal. Both will work but the reveal lining method has the disadvantage of reducing the opening size.
    • CommentAuthorgravelld
    • CommentTimeJan 13th 2018
     
    Thanks!
    • CommentAuthorgravelld
    • CommentTimeJan 13th 2018 edited
     
    I've done some work on the window-in-eaves detail. In the house currently, the tops of the windows are exposed in the eaves on the outer leaf.

    I'm not quite sure what's on the inner leaf. The wall plate continues so there must be some sort of lintel? I assumed that and drew it here.

    Here's a new idea for the eaves insulation. I added a PUR board friction fitted to maintain the ventilation gap instead of cutting EPS on the diagonal. I thought this would be easier to install and also gives slightly better performance at the eaves. Is this better? Also the eaves insulation is now mineral wool because I don't trust filling the space correctly with something more rigid like EPS.

    The other thing of note is what I asked MarkyP about - how the head of the window is secured. I realised I would also need a continuation of the AT layer AND a soffit, so I figured I could just continue a sheet of OSB from the lintel to the fascia board.

    Here's the problem: how to continue the AT layer. If I just have one piece of OSB I can tape the window to the OSB, but I'm guessing the tape can't be simply painted over - that would look crap? Could I:

    - Sandwich the tape between two bits of OSB?
    - (This is what is in the detail currently) Cut the OSB in half, have the tape go over the top, then join the soffit part of the OSB somehow (?)

    This detail still shows a subframe. I am moving away from that idea, beginning to favour brackets.

    Refs:
    - https://retrofit.support/detail/11/
    - https://retrofit.support/detail/12/
      window_eaves_detail.JPG
    • CommentAuthorborpin
    • CommentTimeJan 13th 2018
     
    Posted By: delpradoAm I being stupid or is this over complicated? What is wrong with just attaching it to your reveals, ordering the window with fat frames, and them insulating external and internal reveals. Thats what I plan to do
    If you are using any form of EWI, the recomendation is that the window frame sits within the insulation as it reduces the cold bridging.

    To answer a differnt question re the size of the box, yes it has to be smaller than the opening. Where the opening is block, usually better to have a bit of movement space. For a Timber Frame, the tolerances can be quite tight.

    For inward opening windows, I found the window opening was very tight when the plasterboard was put in. I'd be inclined next time to put 2 pieces of ply in, one smaller than the width of the frame making a slot for the plasterboard (like Velux do). I'd size the openings to allow for this extra all round so keeping same size windows.
    • CommentAuthortony
    • CommentTimeJan 13th 2018
     
    Looks OK I would fix a batten to the blockwork and build a batten frame to carry the OSB soffit. I would cover the osb with 6mm ply where visible

    No insulation over wall plate looks bad
    • CommentAuthorgravelld
    • CommentTime7 days ago
     
    Ooops - forgot to label the white area - it's PUR friction fitted to also provide a guard for the ventilation gap, so there is some insulation.

    I assume a "batten frame" is just a simple frame built with... battens.

    Any thoughts on the air tightness from window head?

    Ah, if I were to build a "batten frame" that could provide a way of sandwiching some tape.
    • CommentAuthordelprado
    • CommentTime7 days ago
     
    Can someone explain to me why all window details shouldnt look like this? Its simpler to do and seems to solve all issues?
      IMG_1989.jpg
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Because not all walls look like that, with insulating plaster on both sides. In general, if you're able to insulate the outside and render it then EWI is a better and cheaper solution. If you're not able to change the outside then you have to just use IWI. In all cases, the window is supposed to be in the insulation layer. Your design looks like the worst of all choices,
    • CommentAuthortony
    • CommentTime7 days ago
     
    It will work though it looks like insufficient wall insulation to me.

    Windows will need massive frames or add-on bits.

    You are also looking the stabiling effect of the mass in the walls
    • CommentAuthordelprado
    • CommentTime7 days ago
     
    Hi DJH

    Presumably though if you use EWI, then you will always be in insulation layer? In the case of IWI you just move it as far forward as possible. If you happen to have it both sides, which is what I am going to have, then I guess it doesnt matter?

    (NB My wall has 70mm eps cavity, but I cannot move the window forward or back, at least in the bays, since they have to take the bay stud load above). Hence the idea above.

    Sorry to hijck thread but in my particular case the plan is to add 50mm wet insualting plater (Baumit DP85) to the outside with a 20mm wood fibre reveal board. On the inside there will be the same reveal board but conventional lime for mass over the top and adjoining wall.
    • CommentAuthorgravelld
    • CommentTime7 days ago
     
    Posted By: delpradoSorry to hijck thread but in my particular case the plan is to add 50mm wet insualting plater (Baumit DP85) to the outside with a 20mm wood fibre reveal board. On the inside there will be the same reveal board but conventional lime for mass over the top and adjoining wall.
    Then, assuming you mean the window is attached to the wall as in your diagram without the IWI, heat will flow from the wall, through the window, to outside.
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: delpradoPresumably though if you use EWI, then you will always be in insulation layer? In the case of IWI you just move it as far forward as possible. If you happen to have it both sides, which is what I am going to have, then I guess it doesnt matter?

    Posted By: djhIn all cases, the window is supposed to be in the insulation layer.
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