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      CommentAuthorheng
    • CommentTimeMar 15th 2017
     
    We're due to fit our new windows soon and I've been wondering about detailing them. The basic problem is that we're not yet ready to fit EWI, so we want a fairly permanent interim (?!) solution for mounting the windows.

    My intent is to mount the external face of the window flush with the wall and affix a drip sill above the window to keep water out of the top gap. The windows are Alu-clad so ought to be fairly weather proof on the outside.

    The reasoning is that this moves the windows as far forward as possible for when the EWI can be done and allows a really simple over insulation detail. In the meantime, the house is pretty normal.

    Around me (Cambridgeshire) one comes across a few houses that have windows mounted flush to the wall with a drip sill above them, so it's not totally unheard of. The weather is pretty dry which might have an influence on the decision.

    I'd be interested to hear of any opinions on this. Clearly I don't want to end up with needing to replace the windows in a few short years!
    •  
      CommentAuthordjh
    • CommentTimeMar 15th 2017
     
    I don't think there's any particular issue mounting windows [approximately] flush with the outside, nor do I know of any particularly 'right' place to mount them. There is an issue with mounting them exactly flush with the outside, of course, which is knowing exactly where the outside is. So you probably want to have an offset to make the inevitable discrepancy look deliberate. Mounting them further outboard tends to minimise water ingress issues but increase lost light/sightline issues.

    Actually, having written that, I'm not clear whether you want to mount flush with your existing walls or flush with the EWI'd walls. The above paragraph assumes the EWI'd walls. If you meant the existing walls then I'd say no, you need them further outboard, because you want the windows in the line of the insulation rather than the solid wall.

    As to how to mount them, I'd suggest an OSB or ply box mounted in the existing rough opening with the windows fitted into it. Cover that with whatever weatherproofs it and satisfies your aesthetic imperative for the interim until the EWI is done.
    •  
      CommentAuthorheng
    • CommentTimeMar 15th 2017
     
    Yes, ideally we'd like to mount them outboard so when the EWI is attached we're inline with the insulation. However, we don't know when that will be, so we're having to compromise and mount them in the existing wall. Mounting them near flush will improve the interface with any future EWI (though yes, we are having to make some thermal compromise wrt future EWI - though the windows are inward opening so we should be able to thoroughly over-insulate).
    •  
      CommentAuthordjh
    • CommentTimeMar 15th 2017
     
    I'd either mount them outboard now, with some temporary waterproof covering, or mount them in a OSB/ply box now flush with the existing walls and then remove them when the EWI is done and reinstall with a slightly deeper OSB/ply box. That's better than leaving them in place with both their cold and warm sides in a thermally conductive layer (i.e. the existing wall).
    • CommentAuthorMarkyP
    • CommentTimeMar 16th 2017
     
    I suspect Heng's windows are sized for the openings and there's not now scope for lining the opening with a projecting ply box.

    I am EWI'ing and debating this very detail in my head. Options I am considering are having windows partly outboard by fixing timber around the outside of the reveal. The window will sit on the timber and the blockwork and be about 60mm into the EWI zone. The jambs I think could be fixed with strap taken back to the inside face of the blocks.

    I dont know how big a deal moving the windows out is in terms of thermal performance. I have seen heat loss diags to show the optimal position is the window sat in the insulation zone. However, in my case, I have an existing cavity insulation layer inboard of the EWI so the cold bridging is less.

    Also, a while back, I recall seeing some metal brackets for this very use case. They were fixed to the outside face of the wall, and include screw adjusters to level the window. I think I recall thinking they looked a bit flimsy, some of my windows are pretty big.

    this blog is worth a read, they used the timber method to outboard their windows: http://www.fourwalls-uk.com/blog/tag/external-wall-insulation/
    • CommentAuthorgravelld
    • CommentTimeMar 16th 2017
     
    • CommentAuthortony
    • CommentTimeMar 16th 2017
     
    I would mount outboard by say 50mm with a drip head and seal all round for air tightness and weather
    •  
      CommentAuthorheng
    • CommentTimeMar 16th 2017
     
    I'm going to do some FEA on this to get a better idea of the effect. We too have a cavity that should help with the bridge effect to some extent.

    And yes, the windows have been ordered!
  1.  
    Hi. Out of curiosity who is making your windows. Thanks
    •  
      CommentAuthordjh
    • CommentTimeMar 16th 2017
     
    Posted By: MarkyPI suspect Heng's windows are sized for the openings and there's not now scope for lining the opening with a projecting ply box.

    He said he's getting new windows.
    •  
      CommentAuthordjh
    • CommentTimeMar 16th 2017
     
    Posted By: hengAnd yes, the windows have been ordered!

    Ah, any chance on modifying the order?
    •  
      CommentAuthordjh
    • CommentTimeMar 16th 2017
     
    Posted By: MarkyPin my case, I have an existing cavity insulation layer inboard of the EWI so the cold bridging is less.

    Yes, I'd guess that makes a difference. The basic idea is that a window has its cold face and warm face very close together so you want it sitting in some region of good insulation to avoid short-circuiting the insulation e.g. through brickwork around the edge of the window.
    •  
      CommentAuthordjh
    • CommentTimeMar 16th 2017
     
    The diagrams on the europhit site seem sensible. The lumps of wood on the fourwalls site look a bit oversized to me. Sure you need something strong enough to rest the window on initially but that's a lot less than some people seem to think. I suppose if your windows are solid timber frames then you're just increasing the size of the thermal bridge they represent and hopefully its over-insulated. But if the windows are triple-glazed then why go with a solid timber frame? PH windows have insulation within the frame so a solid pice of timber alongside them amounts to a short circuit.
    •  
      CommentAuthorheng
    • CommentTimeMar 18th 2017 edited
     
    Posted By: peacebabe68Hi. Out of curiosity who is making your windows. Thanks

    We're going with Katzbeck, an Austrian manufacturer.
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