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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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    • CommentAuthorTimSmall
    • CommentTimeFeb 14th 2018 edited
    I was researching the possibility of using 6-18-4-18-4 for some large triple glazed sealed units (instead of 6...6...6), and I came across this:


    which gives a recommended minimum size. Also the attached - which is DON'T-USE size ranges for triple glazing.

    Both of the sources also recommended the low-e coatings to faces 2+5 instead of 3+5 to reduce heat build-up in the sealed unit.

    ... with the low e on face 3, the middle pane heats up a lot more (twice as much), and this can lead to thermal stress cracking on the middle pane, but also to high gas temperatures, which causes the gas to expand. For larger units the glass flexes to accommodate this, but with one or more short edges, the glass is too stiff to bow outwards, and the additional pressure causes the edge seals to fail.

    The downside of a faces 2+5 low-e coating arrangement, is about a 5% reduction in g value (thermal gains to the room) vs. the 3+5 arrangement.

    Has anyone ever come across this, or had thermal stress cracking with triple glazing?

    Given the UK fondness for small sealed unit sizes, this must happen relatively commonly with UK triple glazing. I'm sure I have at least two units in my house which would appear to break these rules (which are admittedly for Germany - which has slightly higher max temperatures and solar radiation).

    You can apparently reduce this effect by going for low-e glass on faces 2+5, and using a low iron glass as the middle pane - this halves the heat gain on the middle pane.

    Also pretty eyebrow raising was the 80 C + temperatures on the middle two panes of a triple glazed sliding door which is left open (so that the two sashes overlap each other):


    Also see page 17 of https://content.iospress.com/download/journal-of-facade-design-and-engineering/fde1146?id=journal-of-facade-design-and-engineering%2Ffde1146

    ... but then on the other hand, you see SGG glass advising their View Clear with low-e to faces 1+3+5, which is almost the same thermal load on the middle pane as with just 3+5...

    • CommentTimeFeb 14th 2018
    My experiences may be relevant:



    • CommentAuthortony
    • CommentTimeFeb 14th 2018
    No problems with my 3g
    • CommentTimeFeb 14th 2018
    Interesting thread, thanks Tim & Damon.

    Damon, are these all different windows that have broken or have any broken repeatedly? How small are they? The problem with nickel-sulphide inclusions only affects tempered glass, I think. You say the only ones that were toughened or tempered were those required by building regs, but were any of those also ones that broke? Also, the glazing unit spec you give says that the centre pane is toughened. In my experience it's the outermost pane that is toughened for building regs since that's the one that has to resist impacts.

    I saw some Ni-S failures in laminated tempered panels at work. There was a clear focus in the shattering pattern.

    The only failure I've had so far was a glass door - actually half of a pair of french doors - and so a fairly big piece of glass. The outer, toughened pane shattered but we blamed that on a stone thrown up by my lawn mower.
    • CommentTimeFeb 14th 2018
    1) All different windows.

    2) No window at ground level for example has broken, so I may be wrong, or have wrongly transcribed, any specs. In fact I think I resited paying a premium to have some extra toughness on the grounds that I didn't think that people would be hurling stuff at my windows, and I think that that has indeed not been the issue!


    • CommentAuthorIan1961
    • CommentTimeFeb 15th 2018 edited

    Velux have had a well documented failure in some of their double glazed rooflight units with inner panes of float glass breaking. I believe the problem was caused by failure of the edge seals allowing the argon gas to escape resulting in changes in pressure that the float glass couldn't withstand.


    If your issue is not one of edge seal failure the other main suspect would be that the glass units were fitted too tight to the frames with not enough allowance for differential thermal movement between the glass and the frame.

    The problem won't be nickel sulphide inclusions as that only affects toughened glass.
    • CommentTimeFeb 15th 2018
    OK, thanks! Will update my page when I get a chance.

    The perils of self-diagnosis on the Internet...


    • CommentAuthorretrofrit
    • CommentTimeMar 16th 2018

    I had a velux go bang in the bathroom ceiling a few years ago, I think Saint Gobain had the contract to replace and the glazier wouldn't be drawn. at the time I plumped for your second suggestion it had been a really cold night.
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