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    • CommentAuthorgravelld
    • CommentTimeAug 7th 2018
     
    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
    • CommentTimeAug 7th 2018
     
    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
    • CommentTimeAug 7th 2018
     
    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
    • CommentTimeAug 8th 2018
     
    Let's say about a decade until EWI.
    • CommentAuthorgravelld
    • CommentTimeAug 11th 2018
     
    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...
    • CommentAuthorcjard
    • CommentTimeAug 17th 2018
     
    I think the notion of foaming them in as the sole fixing is pretty good. If you’re desperate to install a mechanical fix, you can either drive concrete screws through the frame and into the masonry, on the proviso that it will be possible to remove the window pane and beans, and extract the screws in future.. or you can fit metal legs to the frame, sticking back into the room and fix those to the masonry. Promise that neither will be necessary; once that foam goes off, that window is stuck..

    In terms of sealing the masonry to the window, is that not just the typical point,paint, silicone job?
    • CommentAuthorcjard
    • CommentTimeAug 17th 2018
     
    Ps have you done any assessment of what ewi will save in heating bills, versus the cost of borrowing to get it completed sooner? If you’re going to be paying an extra X pounds per month for the next ten years on heating or interest, getting it done now is better for you and greener?
    • CommentAuthorgravelld
    • CommentTimeAug 17th 2018 edited
     
    Is there a maximum size/weight that's (foam) good for though? We are intending to install a couple of french doors, a 3.5m slider and a large 3x2 fixed light, all 3G.

    Posted By: cjard
    In terms of sealing the masonry to the window, is that not just the typical point,paint, silicone job?
      Render bead rather than silicone I hope.

      I suppose one problem might be scuffing up or otherwise making the outside face of the frame look bad, and subsequent EWI would have to overlap that to hide it.

      Posted By: cjardPs have you done any assessment of what ewi will save in heating bills, versus the cost of borrowing to get it completed sooner?
      Yes, of course. It'll definitely take longer than the period we are in the house (we're hoping to get 20+ more years out of this house), so there's no financial benefit or repayment.
    •  
      Posted By: gravelldYes, of course. It'll definitely take longer than the period we are in the house (we're hoping to get 20+ more years out of this house), so there's no financial benefit or repayment.

      Over here the ROI on EWI is (unfortunately) the same. My son is doing EWI on his house and even with the government interest free loan over 20 years the numbers don't stack up. What did stack up was draft proofing and boiler change.

      The reason he is doing the EWI is planning permission, he is doing an extension later and the PP will require the whole structure to come up to current standards and an interest free loan now makes sense. (plus comfort levels in the summer).
      • CommentAuthorcjard
      • CommentTimeAug 18th 2018
       
      If you’re really concerned about the weight then I’d use this https://www.abbuildingproducts.co.uk/GBR/touch-n-foam-15-kit-complete.html rather than one of the “large aerosol with a plastic straw on top” ones -the foam the two part kits make is much more uniform in structure and rigid..
      ..but remember that there isn’t much shear force from a fixed pane or slider, certainly not from the window itself, only from the wind blowing on it. On the French doors, a single mechanical fixing vertically somewhere above the hinge line might be advantageous but even then it would take a concerted effort to wrench the foam and the render out of the way

      Yes, was thinking a bead of render (pointing) or application of rendered area would hide the foam edge, painted, then a bead of silicone seals the render to the window

      I’d assumed he EWI question was whether to do it now with a loan or in ten years having saved up - wasn’t really counting the capital cost of the EWI itself, I was assuming you were committed to it regardless of ROi, and hence was just wondering whether the savings offset the interest to give you 20years of comfort vs 10years of discomfort plus 10years of discomfort (if that’s even what it gives you?)..
      ..though I do wonder why you’re bothering if it won’t ROI in the 20years you plan to use the house!
      • CommentAuthorgravelld
      • CommentTimeAug 20th 2018
       
      Cheers. Got a couple of meetings with window suppliers this week so will discuss then. I'm committed to it in my head as the right thing to do, just that resources (overwhelmingly financial but also a bit mental wrt the complexities) don't stretch to it right now.

      If the money wasn't in windows it would be in the markets, 7% in real terms on average, no contest really.

      I'm bothering because I want a more comfortable, less complex and more maintainable house (for my family).
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