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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentTimeMar 28th 2018
    Posted By: TimSmallThe location of airtightness layer is not from wufi modelling, but down to the risk of convection (e.g. around defects in the insulation) leading to condensation on the inner side of the airtightness layer
    Ah fair enough. Insulation convection paths can happen anywhere, inboard and/or outboard of the airtight layer, and are liable to cause condensation anywhere. Though such condensation is most likely on the colder outboard side, if by slight chance it happens on the inboard side, it's going to be extra-problematic.

    If confident that inboard insulation is convection-path free, then airtightness can go anywhere, I'd say? Even failing that proviso, in view of that 'slight chance' you're covering, I don't think I'd make it a golden rule. What I do suggest, is to keep all organic material out of the outer 40-50% of the sandwich, as that's where any condendsation, whether permeation- or air-borne, is going to happen.

    On the other point, I don't see why symmetrical vapour permeability across the sandwich is important, as successor to the old idea that it should be 3:1 or 5:1 asymmetrical?
    • CommentTimeMar 28th 2018 edited
    Posted By: Ian1961I read that the problems at Dartington were caused by leaks rather than condensation issues.
    The only info on that is what the lay press first reported; since then in litigation and poss NDAs, no-one's going to give us the benefit of what really happened.
    • CommentAuthorTimSmall
    • CommentTimeMar 28th 2018 edited
    Posted By: fostertomI don't see why symmetrical vapour permeability across the sandwich is important, as successor to the old idea that it should be 3:1 or 5:1 asymmetrical?

    I suppose what I was trying to get across is that this type of arrangement (or with more resistance towards the inside) is more likely to "just-work", without modelling to check. That having been said, it doesn't really work with rain-wetted materials on the outside which are subject to capillary action - but really just for rain-screened constructions, or those with a capillary break towards the outside (e.g. EPS EWI etc.).

    Modelling has made me think "hmm, that's interesting, I wasn't expecting that, and won't be doing it that way then..." more than once.
    • CommentTimeMar 28th 2018
    Yeah, I miss it - haven't had access since my initial 16wk freebie!
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