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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Fundamentals: Roof Ventilation Gap Above Insulation

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1 to 13 of 13

  1.  
    • CommentAuthortony
    • CommentTimeNov 21st 2012
     
    Who decided on a gap of 50mm for ventilation above the insulation?

    Would 49mm be OK? or 25mm? or less?

    The extra depth of insulation could save us a boat load of energy if it could be halved to 25mm.

    I would be happy with a careful 20mm on my own house

    Or was it someone at a desk in the 80's who did decide this?
    • CommentAuthordocmartin
    • CommentTimeNov 21st 2012
     
    Was it to allow for the sarking membrane draping into the space and still permit sufficient ventilation?
  3.  
    I believe it is a rule of thumb derived form Manufacturers BBA Certification - this probably from Laboratory tested values - specifically rigid board in cavity and cold roof applications. It's also about ensuring an adequate surface area of timber (in roofs for example) for ventilation of moisture.
    • CommentAuthortony
    • CommentTimeNov 21st 2012
     
    Good points mike but the second one was more relevant in the old days when 50mm started when sarking was not permeable. Now it allows moisture through, I never saw wet or damp roof timbers in any roofs that weren't leaking or unventilated
    • CommentAuthorCav8andrew
    • CommentTimeNov 21st 2012
     
    There was some research documented in a USA Building Science Research paper that suggested that 3/8th" pretty much worked in all circumstances.
    • CommentAuthorTimber
    • CommentTimeNov 21st 2012
     
    BS 5250? I can't remember if it does state depth for a pitched roof (I think it does), but it certainly states dims for gaps in cold flat roofs.
    • CommentAuthortony
    • CommentTimeNov 21st 2012
     
    10mm sounds like the Us have fewer belts and braces than us or sense which is a rarity.
    • CommentAuthorCWatters
    • CommentTimeNov 22nd 2012 edited
     
    Posted By: docmartinWas it to allow for the sarking membrane draping into the space and still permit sufficient ventilation?


    That's my understanding. The membrane should drape 1" to stop water pooling above tile battens.
    • CommentAuthorCWatters
    • CommentTimeNov 22nd 2012
     
    Forgot to mention that ventilation is only required if the membrane isn't vapour permeable but you may have to convince the BCO. With a permeable membrane you still have to allow for the drape or use counter battens. Some makers don't like the membrane to be in contact with the insulation while others (Kingspan?) say it's ok.

    Perhaps worth a look but note they use "warm roof" to mean "warm loft" not "warm rafters"..

    http://www.ecobuild.co.uk/var/uploads/exhibitor/1228/roof_ventilation_feb_2007_2.pdf
    • CommentAuthorCWatters
    • CommentTimeNov 22nd 2012 edited
     
    Checked and kingspan say no ventilation or drape gap below membrane is required for their nilvent product..

    http://www.kingspaninsulation.co.uk/getattachment/dcc48e8c-6ccb-4ebf-bddd-d35b87af0b48/nilvent-Breathable-Membrane.aspx

    See images page 2.
    • CommentAuthorCav8andrew
    • CommentTimeNov 22nd 2012
     
    Surely there are now plenty of examples of breathable membranes being laid directly onto OSB, ply or boards, then counter battened and battened and tiled. The 3/8th rule, if I remember correctly, referred exclusively to breathable membranes.
  12.  
    Posted By: CWattersSome makers don't like the membrane to be in contact with the insulation while others (Kingspan?) say it's ok.
    Most manufacturers allow use of their breather membranes in direct contact with sarking boards or insulation. In the BBA certificate these are referred to as "non-tenting" &/or non-woven polyolefin membranes. By non-tenting they mean that they don't leak water when in direct contact with a surface in the way a woven tent fabric does.

    David
    • CommentAuthorCav8andrew
    • CommentTimeNov 22nd 2012
     
    When I discussed this with our breathable membrane manufacturer, they had a slight preference for tented install (for the elevation of penetration points reasons) however as they pointed out, if your membrane is required to provide an airtight barrier it is harder to tape the joints to an airtight standard when draped.

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