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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Housing - Renovation: Condensation Risk Analysis + my roof make up

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    • CommentAuthorDubdubz
    • CommentTimeDec 17th 2015
     
    As suggested here's my new thread....I have been looking at the wealth of info on here and it is truly amazing what is discussed [perhaps I missed my vocation!]



    I downloaded build desk but I don't it find intuitive and I'm impatient?? I ha a look at Wufi and 1 other that wouldn't load - dewpoint3

    I'm building a hybrid roof

    I'm not sure where to put a VCL or a breather layer ?? I'm thinking this is the current as drawn

    EPDM
    18mm Ply deck T&G [glued along edges]
    130mm Celotex [couldn't find this as a material in build desk because I'm stupid]
    6mm Ply
    80mm Celotex between
    215x47mm C24 rafters at 400 centres
    VCL
    15mm plasterboard

    the odd thing is in my uneducated brain - the dewpoint must be directly on the top of insulation surface so if you put a EDPM directly to the insulation isn't the dewpoint UNDERNEATH the EDPM and on top of the insulation? Or in fact underneath any material above the insulation surface?

    I've seen posts where a breather membrane is recommended, but does this need an air gap to work? so if I did the below scenario, this protects the ply deck and the joists, but if there is any condensation how does it escape?
    *adding breather

    EPDM
    18mm Ply deck T&G [glued along edges]
    130mm Celotex
    >>Breather membrane<<
    6mm Ply
    80mm Celotex between
    215x47mm C24 rafters at 400 centres
    VCL
    15mm plasterboard

    lastly is this all predicated on fresh air or air leakage in between layers due to construction gaps not being sealed ??



    Perhaps I should actually do this

    EPDM
    18mm Ply deck T&G [glued along edges]
    40mm battened air gap
    Breather membrane [taped]
    130mm Celotex
    6mm Ply
    80mm Celotex between
    215x47mm C24 rafters at 400 centres
    VCL
    15mm plasterboard

    cheers Guys and Girls

    Darren
    • CommentAuthorEd Davies
    • CommentTimeDec 17th 2015 edited
     
    Posted By: DubdubzI've seen posts where a breather membrane is recommended, but does this need an air gap to work?
    Yes, it needs a ventilated air gap to work.

    “Breather” is a bit of a misnomer as it only “breathes” if the context (materials each side of it) are such that it happens that water vapour flows one way at some times and other ways at other times (and you think there are creatures that breath water vapour).

    All it actually is is a barrier to stop liquid water and large amounts of air getting into a material without stopping smaller amounts of gas, typically water vapour, from getting out. It won't do anything magic to make water vapour move, it just stops it a lot less than other membranes intended to stop liquid water and bulk air would likely do.

    I'd imagine that your last version, with ventilation of the gap, would be the best bet.
    • CommentAuthorEd Davies
    • CommentTimeDec 17th 2015
     
    You have 215 mm rafters with 80 mm Celotex between. With that much space available, couldn't you get the same insulation resistance for less money using something other than Celotex?

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