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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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General: Thermal bridge - Window Pillars

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  1.  
    Cavity Wall Property - 2 sets of large windows with a solid brick pillar in between each pair, photo below:

    https://drive.google.com/file/d/12ZZsoLmjjSMoA4AW8HBQ_oQzmi9waWuE/view?usp=drive_link

    Obviously this is a thermal bridge and we'd like to engineer it out.

    Option A
    Remove pillar - replace with a structural beam, disc out all cavities around window openings and fit cavity closures. Then EWI.
    Appears to be messy, labour intensive, minimal gain?

    Option B
    Just install new windows outside of external skin, EWI over pillar and walls - and be done. Perhaps not as thermally efficient but cheaper, faster, less mess?
    •  
      CommentAuthorfostertom
    • CommentTimeOct 6th 2025 edited
     
    The latter - nothing inferior about it at all. With EWI, both walls and pillars become entirely part of the internal environment. Their conductivity is almost negligible within the EWI'd wall sandwich - whether they're there or somehow miraculously removed, makes no more difference than adding or subtracting a piece of furniture, to the thermal performance.

    Always assuming the cavity isn't somehow a conduit for outside air into the interior - now there's a thermal bridge for you!
    •  
      CommentAuthordjh
    • CommentTimeOct 6th 2025 edited
     
    Posted By: VictorianecoJust install new windows outside of external skin, EWI over pillar and walls - and be done. Perhaps not as thermally efficient but cheaper, faster, less mess?
    Sounds like a plan.
  4.  
    Great, I thought that would be the case

    Now it's a case of final decision of tilt and turn or outward opening.

    And then double or triple glazing ?
  5.  
    Tilt and turn every time for me.

    2G or 3G - budget constraints, exposed location, noise mitigation and window size all come into the decision making. For a large window perhaps 3G is justified especially in an exposed location. I usually go for 2G unless other factors come into play e.g noisy environment or exposed large window.
    • CommentAuthorLF
    • CommentTimeOct 7th 2025
     
    I noted your recent post about aluminium - did you consider good UPVc ?

    I've installed some 2g and 3g glazed upvc this summer - very pleased with result. We are getting to end of the work now.

    Even get condensation on outside of double glazed - need to thermal scan them when it is a bit colder.

    Just touching the glass and reveals it is much warmer than rest of house which is reasonable 20 odd year old upvc 28 mm double glazing without thermal edge enhancement of modern stuff, they are softcoat.

    I am sure I got supplier from on someone on here, modernupvcwindows. I visited them first and have been pleased. Good value.

    One window 2.2 m wide triple glazed with a small opening window on the end that opens outwards - see photo.
    Installed so some of frame covered from inside - I added insulation then had it skimmed.

    We have a sliding window (actually door) that is 2.8 m by 1.7 m with a window seat under. That had to be double glazed.
    a triple glazed full length glass door 1m wide. Very good quality - friend asked if it was Velfrac.
    It is black and when open gets the sun and needed glazing packers adjusting a couple of times. Really air tight frame.

    We cut the bricks that bridged the cavity and insulated the cavity, the windows bridge the cavity about 30 mm in.

    A lot of work - so fitting on outside in your new insulation layer sounds potentially a lot better.

    I will add some pickies later struggling to add at the moment.
  7.  
    Yeah I had them on our list, and prices seem good. But wanted to see in person
    • CommentAuthorLF
    • CommentTimeOct 10th 2025
     
    Kitchen window is triple glazed. Use insulated board around internal reveal.

    Enough space to get beads off but reduces the impact of frame. (and makes it warmer) Insulated boards (backer type) fixed on with flexifoam on top of the cavity closures. Many I could only get 20 mm thick board on. It skims over well - plasterer was happy.

    Air tightness tape from window to on side of cavity closure and from other side of cavity closure to wall.

    Window seat window is sliding door - double glazed. Fixed as above.
      good spec upvc windows.jpg
    •  
      CommentAuthorfostertom
    • CommentTimeOct 10th 2025 edited
     
    Posted By: LFnough space to get beads off but reduces the impact of frame. (and makes it warmer)
    Always good to 'bury' the width of the fixed frame, in as much insulation thickness as possible, at the jambs, both internally and (if you're doing EWI) externally. It makes a surprising difference to the effective overall insulativeness of any window.

    Or, if you can't do the 'burying' with insulation (because it's all made of plasterboard/backer board etc internally, or of cladding material externally) it at least hides the excessive width of most modern fixed-frame sections.
    • CommentAuthorLF
    • CommentTimeOct 11th 2025
     
    Insulated backer board used for reveals.
    2 layers of 20 mm in some places.
    One sheet of 10 mm also used.

    Not as rigid as plasterboard so a bit more fiddly to get flat.

    Blue flexifoam used to stick it in place.

    Pretty hard to burn, goes out quite quick.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 11th 2025 edited
     
    Got a link for your preferred insulated backer board?

    Given the narrow width of reveal boards, I'm surprised you find such narrow plasterboard strips significantly easier to get flat? On the other hand, in an old house, maybe we''re looking for ways to reproduce a bit of waviness, tho plasterers will find a way to make it severely flat, whatever you do!
  12.  
    Looking at ~120mm EWI for my project. Considering the depth both sides of the windows, it actually reduces the viewing angle of the outside somewhat!

    Even though the windows would be mounted within the EWI layer the depth from outside to insure becomes quite large.

    Has anyone noticed this particularly on smaller windows ~2m2 of obscuring the angle of view?

    Really starting to doubt if the labour, cost and benefit is really that great as I'm assuming heat pumps will likely improve all even further in the future
    •  
      CommentAuthordjh
    • CommentTimeOct 11th 2025 edited
     
    Posted By: fostertomtho plasterers will find a way to make it severely flat
    I was very pleased when my plasterer asked me how smooth/flat I wanted the finish :bigsmile: Smooth but not completely flat was my answer.
    •  
      CommentAuthordjh
    • CommentTimeOct 11th 2025
     
    Posted By: VictorianecoEven though the windows would be mounted within the EWI layer the depth from outside to insure becomes quite large.

    Has anyone noticed this particularly on smaller windows ~2m2 of obscuring the angle of view?
    Our windows are mounted nearer the outside for that reason, as will be yours. Deep window boards are a useful feature; we can sit on one in the living room that is low enough.

    Our interior reveals are curved back a bit, which helps the view and lightness of the openings. The exterior can be curved back as well but ours was kept right-angled for aesthetic reasons.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 11th 2025 edited
     
    Posted By: Victorianeco120mm EWI for my project
    Why only? you'll save £ peanuts, as insulation boards' cost is the least of it, you're not constrained by narrow side path, and eave and verge overhang is generous.
  16.  
    That would get me to Enerphit Levels of ~0.15 for the walls

    Why go more? reduce the angel of view from the windows, how many days a year heating will it really save?
    •  
      CommentAuthorfostertom
    • CommentTimeOct 12th 2025
     
    Posted By: VictorianecoThat would get me to Enerphit Levels of ~0.15 for the walls
    That must be because you have wide cavities well CWI'd? I must have missed that. If so, then yes, 120.
  18.  
    The cavities are 80mm and will be filled with Graphite Beads,

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