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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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General: Installing windows in EWI layer?

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    We're going with Aluminium windows, undecided if these are going to be inward opening or tilt and turn at this point.

    We are looking to add them to the EWI layer of our cavity wall property.

    We are thinking:

    - Remove 25mm of render all around the existing window opening
    - Make the windows 20mm wider all around than the window opening
    - Mount then externally with brackets
    - Use compactfoam below window to support them horizontally?
    - Then use air tightness tape around inside and outside the windows?
    - Render outside using sand/cement
    - Then EWI?

    Is there a better detail than this?
  2.  
    If you are mounting the windows on brackets how will the compact foam 'support' the windows?
  3.  
    Sorry brackets to sides of windows and the compact foam on the horizontal?
  4.  
    OK
    I've used brackets on the bottom and the sides and then the usual EPS up to the window.

    Will the compact foam just spread the load onto the EPS which will then still be carrying the weight of the window? (Other than the side brackets which will be in sheer)
  5.  
    Not sure? I was just going off something I read on here previously?
  6.  
    Compactfoam is dense enough to screw into - we bought stainless steel threaded hooks which we screwed into the compactfoam and then cast into our (new) slab. This was based on the approach I read about here:

    https://www.21degrees.com/compacfoam/

    See their photo below:

    Not sure it will apply if you are retrofitting but perhaps you can cast something locally to counterbalance the weight of the windows?
      21-degrees-compacfoam-large-2-400x510.jpg
    •  
      CommentAuthordjh
    • CommentTimeSep 29th 2025
     
    I'm not sure now whether we're discussing doors or windows? Check whether the windows are designed to be supported at the sides or the bottom and then plan accordingly. We have one large, wide window that is fixed with straps at the side like all the others but also rests on the OSB box at the bottom, largely as a matter of convenience whilst we were installing it.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 1st 2025 edited
     
    If Compactfoam is really able to be structurally screwed into, apparently able to provide frictional restraint by compression, and able to a hold a long-screw straight where it passes through, then it eases many of the detailing problems that arise with thick EWI-type insulation (whether true EWI on a wall, or EWI-like insulation applied over a timber roof slope).

    Trouble is, the modern trend to provide neither a technical design manual, nor any equivalent info online. Till the recent past, serious product manufacturers published everything, tho some better than others. "Request a brochure" isn't it. Now, and in Compactfoam's case, to get any further you have to "Request a quote" or Contact them - a sure prelude to listening to a load of sales chat. Life's too short.
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      CommentAuthordjh
    • CommentTimeOct 1st 2025
     
    Tom, I'm not sure where you looked but google showed me data that includes screw pull-out numbers and suchlike, more data than I need certainly.

    https://odoo.21degrees.com/shop/compacfoam-74#attr=105
    https://www.wiki-compacfoam.info/wiki/en:brandverhalten-von-compacfoam
    •  
      CommentAuthorfostertom
    • CommentTimeOct 1st 2025 edited
     
    I take it back - good stuff there, under 'download brochure' - too quick to assume it was same sales blurb as 'Request a Brochure' elsewhere.

    Straight off, a pic of Compactfoam similar to my use of chunky timbers lately, like 35x65 and 32x32, as 'picture frame' surround/mounting for window frames outboard in EWI (instead of flimsy ply 'picture frame' or metal brackets). If the timber's not too bad for that purpose at U1.0, Compactfoam's better at U0.54, tho still not matching the U0.38 of the EWI it's embedded in.
    • CommentAuthorwookey
    • CommentTimeOct 8th 2025 edited
     
    Yes. I think your plan for window mounting sounds good.

    As belt+braces I would use illbruck permanently flexible airtightness foam as well as tape (That's how internorm fitters did my windows, which are otherwise much like yours, but not oversized).

    You can now get Almavert from Partel in Ireland which has very similar properties to compacfoam but is cheaper and is recycled PET rather than virgin polystyrene. I've not actually cheked the LCAs but I assume it's significantly better.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 8th 2025
     
    Un-whisper that, wookey - it's too good to not be seen (I seem to have some special privilege so I see these accidental self-whispers)

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