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Housing - New Build: Roof detail - measuring breathability of sarking membrane

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  1.  
    • CommentAuthorRobinB
    • CommentTimeMar 16th 2010
     
    Roofing Spec says " Breathable sarking membrane must not exceed 0.60 MN.s/g "

    Membrane we plan to use says "Water Vapour Resistance (BS 3177) 0.19 Mns/g" so is that better or worse? I'm not sure!

    thanks for info
    RobinB
    •  
      CommentAuthorali.gill
    • CommentTimeMar 16th 2010 edited
     
    Poor spec in the first place..
    BS5534 CoP for sarking and tiling recognizes two types of membrane:
    High vapour resistance (HR) underlay has a water vapour resistance greater than 50MNs/g which prevents transfer of water vapour.
    Low vapour resistance (LR) underlay has a water vapour resistance not more than 0.25MNs/g which allows the transfer of water vapour

    So if number is greater than 0.25MNs/g then its not considered within the remit of a 'breathable' or 'vapour permeable membrane'.
    I'd question the integrity of anyone that uses the term breathable in product specification, as the discussions on here have shown it means different things to different people and encapsulates three distinct areas of dynamic vapour 'activity'.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 16th 2010
     
    MN.s/g tells us how many megaNewtons of pressure is reqd to produce a vapour flow of 1 gram per second (which wd be an extreme pressure producing a stupendous flow rate!).
    So a smaller no of megaNewtons means it's less resistive.
    Note that this is resistance, not resistivity, i.e. the thicker the membrane the higher the resistance.
    Resistivity, which is resistance per metre thickness, is in MNs/gm (in which 'gm' doesn't mean gram, but 'gram x metre')

    There's an important role for membranes of low-to-medium resistance, like OSB, which at typical 9mm thickness has vapour resistance of around 1.95 MNs/g. As rule of thumb, breathing constructions require a ratio of inboard to outboard vapour resistance of about 5:1 (more resistive inboard). So 1.95 inboard OSB, grading to 0.25 or less outboard, is ideal. Plain plasterboard is too unresistive for such an inboard 'resistor' purpose; vapour-check plasterboard is much too resistive.
    • CommentAuthorRobinB
    • CommentTimeMar 16th 2010
     
    I'm trying to allow transfer of water vapour. The O.25 membrane is outboard. So that sound OK.
    thanks

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