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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorgravelld
    • CommentTime7 days ago
     
    Regular readers will have read how I've had to scale back my retrofit project.

    We're still going to buy some decent windows, just can't afford the EWI to fit them inside!

    If we're installing windows into the external leaf of an existing cavity wall, what best practices can we follow?

    The main thing I'm thinking is to establish the AT layer at future AT layer by taping into external render.

    I'll also have render beads against the frames for the re-rendered external reveals.



    In terms of the actual placement of the window, there are two options:

    - Install in same position and try to get some way to move the window when we do EWI
    - Install flush to the edge and when EWIing just go over the top

    I like the first one in the sense we can eventually get the best performance plus it will look better short term, but I don't know how realistic moving windows is.



    We currently have mould on our reveals both on the window frame and the wall. I suspect the former will be solved by the new windows, but the latter probably not - if anything it might be worse.

    Is it worth fitting a small amount of insulation on the internal reveals up to the frames to avoid this?
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: gravelldThe main thing I'm thinking is to establish the AT layer at future AT layer by taping into external render.

    You will also need to create (tape) an internal vapour barier, at least until you install your EWI, since otherwise there will be a path from inside to the cold external AT layer.

    I like the first one in the sense we can eventually get the best performance plus it will look better short term, but I don't know how realistic moving windows is.

    I would think moving the windows will be very doable, as long as you plan in advance, possibly compromising the current performance or appearance to make the reinstallation easier.

    We currently have mould on our reveals both on the window frame and the wall. I suspect the former will be solved by the new windows, but the latter probably not - if anything it might be worse.

    I'm not clear why you think the situation will be worse? If the inside of the window will be warmer then surely the reveal will be too? If it is a problem, then yes some insulation in the reveal may sort it. It's the sort of thing that might need to be discarded and redone when you do the EWI.
    • CommentAuthorgravelld
    • CommentTime7 days ago
     
    Thanks djh, great stuff.

    Posted By: djh
    You will also need to create (tape) an internal vapour barier, at least until you install your EWI, since otherwise there will be a path from inside to the cold external AT layer.

    Great stuff, thanks.


    I would think moving the windows will be very doable, as long as you plan in advance, possibly compromising the current performance or appearance to make the reinstallation easier.
    I thought so to, although one installer I talked to screwed his face up at that... although this was a more 'traditional' uPVC installer who I was getting to quote to judge the value comparison between 'normal' uPVC and the factory built windows we normally talk about here.


    I'm not clear why you think the situation will be worse? If the inside of the window will be warmer then surely the reveal will be too? If it is a problem, then yes some insulation in the reveal may sort it. It's the sort of thing that might need to be discarded and redone when you do the EWI.
    You might be right; I assumed there'd be a higher AH and so it might concentrate where it's colder, but given the proximity of the window and wall the former may well help the latter retain heat.
  1.  
    Posted By: gravelld- Install in same position and try to get some way to move the window when we do EWI

    Depends how long you think it will be between installing the new and wanting to move them. I recall hearing that the cowboy PVC window installers would just put them in the hole fixed with squirty foam only. If you did this then it would be a fairly simple job to cut around the foam, remove the window and replace them properly in the EWI layer.
    • CommentAuthorgravelld
    • CommentTime7 days ago
     
    Let's say about a decade until EWI.
    • CommentAuthorgravelld
    • CommentTime4 days ago
     
    About the vapour barrier - what are the options?

    Just using a tape?

    Can we avoid that and repaint with a paint vapour barrier (if one exists). It's wet plastered internally up to the window.

    Should we board the reveals with a thin layer of EPS to avoid more mould on the inside, or should the extra performance of the new windows (Uw will be 1/3 that of the old windows) warm the wall enough to avoid that? I'm just thinking, if we have to make it good anyway...
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