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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorBowman
    • CommentTimeNov 10th 2017
     
    Okay next dilemma,

    New timber frame first floor and roof on top of existing block ground floor, R34 batts between timbers and 120mm board outside - a la EWI.

    I was planning on fitting a reflective breather membrane (Tyvek Reflex or similar) but if the boards are impermeable what is the point?

    We have MVHR.

    Any thoughts most welcome.
    • CommentAuthortony
    • CommentTimeNov 10th 2017
     
    It is completely irrelevant whether you have MVHRor not in relation to the question.

    Inside you should have a vapour barrier and outside any membrane will do,
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: BowmanI was planning on fitting a reflective breather membrane (Tyvek Reflex or similar) but if the boards are impermeable what is the point?

    The point of reflective or breather or any membrane at all?

    If the boards are impermeable, are the joints? i.e. are you going to make them into an air barrier? And if so, what is the construction on the inside?

    What about the membrane? Is that going to be an air barrier or not?

    Basically, any moisture that is in the construction needs to be able to get out. That includes moisture in the timber when you buy it and/or rain on it before it is covered. It also includes any moisture that gets into the roof after construction is complete, through holes, broken tiles, whatever.

    So either the external membrane needs to be loose lapped or it needs to be breathable. And either the entire construction needs to be permeable or the interior membrane needs to be (semi-) permeable as well.
    • CommentAuthorBowman
    • CommentTime7 days ago
     
    Basically as I see it completely wrapping the house in celotex will create an impermeable barrier making a breather pretty pointless
  1.  
    120 boards as per EWI is surely not Celotex (or shouldn't be)? I had assumed something like EPS.
  2.  
    We did something similar a number of times without a breather membrane. We did use an OSB board in the middle of the wall as an airtightness layer.
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    • CommentAuthorcjard
    • CommentTime7 days ago edited
     
    If it's impermeable and covers the same extent as the reflex, then the reflex is pointless. Impermeability with sheet insulation is a function of the quality of installation. Are you certain it's impermeable?
    • CommentAuthorgoodevans
    • CommentTime6 days ago
     
    I'm nervous about vapour impermeable - there are so many ways by which liquid water ends up in the wrong place - it then runs and pools concentrating repeated wetness in a specific (but unpredicatable) places. - if you have structural wood at that location you have a problem (even masonry doesn't like it).

    From what I've read vapour barriers (vcl's) might (or might not) help prevent internal moisture getting into the roof - depending on how many holes there is in it - or it might hinder evaporation to the inside if water is penetrating from outside. My preference would be not to go for no vcl internally but use a breathable air barrier internally this slows down drafts carrying moist internal air to cold parts of the roof) but this flies in the face of most roofing details and bba certificates. I think this is mainly how vcls work - they reduce moist air exfiltration - but the risk is water on the wrong side of the vcl can not escape to the house where ventilation/MVHR can deal with it easily.

    By all means use a vapour impermeable roof felt but loose lap it and ensure you have a small gap underneath it to allow pooled water to evaporate and find a path out. Or use a vapour permeable felt which allows you to tape it as an air barrier, and also to lay it directly on a surface such as celotex or sarking.
  3.  
    What about wood-fibre externally? I assume there's battens and cladding outboard of the membrane? If you used wood-fibre (and assuming that R34 - of which I have no knowledge - has a degree of breathability) then perhaps an intelligent membrane internally and a basic breathable membrane externally might do. Alternatively, if you lime render the wood-fibre, you don't need the cladding or an external membrane.
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    Posted By: goodevansMy preference would be not to go for no vcl internally but use a breathable air barrier internally this slows down drafts carrying moist internal air to cold parts of the roof) but this flies in the face of most roofing details and bba certificates.

    The product designed for that job is a semi-permeable vapour barrier - Intello or similar.
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