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Green Building Bible, Fourth Edition
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    I was called to look at an extension with very badly cracked render. The lay-up, according to the original builder from 10 years ago, is:

    12.5 plaster boards (I do not know whether they are foil-backed)
    18mm OSB
    150 x 50 timber filled with kingspan
    18mm OSB
    Breathable membrane
    Heraklith 50mm boards.

    The render appears to me to be sand/cement, and it seems to be painted in a non-permeable paint.

    The above is enough of an issue, but I am also not sure I am comfortable with the 2 layers of 18mm OSB. Even if the plasterboard is not foil-backed, the internal OSB will perform some sort of vapour-check function. The existence of a 2nd layer of 18mm OSB on (except for 50mm Heraklith, which is not desperately low in lambda value) the cold side worries me a bit. If the inner layer of OSB lets some w.v. through, is there not a risk of interstitial condensation on the outer OSB layer.

    I have not spent long thinking this out, so you may see immediate holes in my thinking, but any criticisms or corroborations are welcome. (I have not opened up the wall yet, to see whether there is any evidence that any of my dire prognostications have come to fruition! That comes later!

    Thanks in advance.

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    • CommentTimeJan 20th 2019
    Unless the internal OSB is taped, or fully glued at the edges, it won't act as a vapour barrier in any case. Either possibility will need to have been done deliberately, and both need to be done carefully and with the right materials if they're going to last. So I'd say there's a minimal chance there's an effective vapour-barrier.

    In which case, yes, I'd expect there might be some condensation on the inside of the outer OSB, and also on the inside of the render. So problems might manifest especially towards the bottom, like your first picture.

    But there's no way to be sure without opening it up a bit. A moisture meter will be useful, I expect.
    Thanks djh. Yes, you are right re the OSB - it should perhaps be treated as 'guilty' (untaped) until proven 'innocent' (taped to within an inch of its life).

    I will need to open it up, yes, and a moisture-meter will, I think, be critical. I wonder also about the rigid insulation trapping moisture against the studs as the water vapour gets towards the cold side.

    When you talk about the moisture at the bottom, are you just referring to it running down and concentrating at the bottom? I suspect that part of the damp at the bottom is from rain splash-up. The decking is, apparently, 'for the chop', so we will probably have a softer and lower surface before too long.

    Thanks for your input.

    • CommentAuthortony
    • CommentTimeJan 20th 2019
    SIPs are insulation between two layers of OSB!

    I suspect no vapour barrier

    I would like to have seen a ventilated void behind the render.

    I suspect damp in the form of condensation is forming in layer behind the render

    It is horrible, the water vapour on its own trying to escape from inside will take off the paint
    Thanks Tony,

    I take your point re SIPs, but I think the difference is that SIPs are likely to be very 'tight' and I suspect this is not.

    It may be possible to engineer a ventilated vid, given that the Heraklith is not contributing much to the overall insulation value. If we removed the 50mm Heraklith, then battened and covered with 20 or 25mm Heraklith or similar, that would give us a 25mm void. I take it you are suggesting that this would in effect act as a 'vapour drainage channel', where vapour could be taken away by the breeze or, in the worst case, liquid water could drain to the bottom of the void.
    Oops - posted that twice!
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