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How can I tell if my felt is 'breathable' or 'non-breathable'

  • 12-11-2012 11:12AM
    #1
    batman1
    Registered Users, Registered Users 2 Posts: 614 ✭✭✭


    How do I tell if the roofing felt on the house in breathable or non breathable?

    All I can tell is that it is black!

    Not an expert but the house was built in 2002.

    Is there a year after which it should be breathable?


Comments

  • #2
    engineermike
    Users Awaiting Email Confirmation Posts: 280 ✭✭engineermike


    batman1 wrote: »
    How do I tell if the roofing felt on the house in breathable or non breathable?

    All I can tell is that it is black!

    Not an expert but the house was built in 2002.

    Is there a year after which it should be breathable?

    Is it felt ? or breather membrane ? If you attach a picture or details of the manufacture stamp on the layer - I might be able to tell you.

    The requirement depends on the construction detail of the roof design - i.e. warm roof / cold roof etc.
    mike f


  • #3
    batman1
    Registered Users, Registered Users 2 Posts: 614 ✭✭✭batman1


    It's felt and a cold roof structure. Ill try and get a picture but not easy to get.


  • #4
    sinnerboy
    Closed Accounts Posts: 5,433 ✭✭✭sinnerboy


    If the felt feels stiff and "tarry" and has a noticeable square grid about 10mm spacings it is not breathable.

    If it feels light and canvass like it is probably breathable.


  • #5
    engineermike
    Users Awaiting Email Confirmation Posts: 280 ✭✭engineermike


    Hi, in agreement with sinner here
    follow his test - but sounds like non breathable,
    If the felt is an older, non-breathable felt, then a gap of 50mm should be left between the insulation and the felt to prevent the formation of condensation on the wood, which may lead to rot. The ventilation from the ventilation openings at the eaves should be routed up behind the insulation.
    What is the cause issue for your post, if i might ask OP - this might help determine if its either / or .
    mike f


  • #6
    batman1
    Registered Users, Registered Users 2 Posts: 614 ✭✭✭batman1


    Thanks, I think it's the non breathable one I have then.

    Was exploring sprayfoam insulation and had an engineer friend look into it for me. There's only one product certified in UK/IRL for spraying onto non breathable felt but is expensive. If it was breathable, I would have more options. Trying to sort out insulation etc on a poorly built dormer. Have already spent loads on insulation/foils etc but not much improvement. My problem lies with airtightness, not insulation so much. It's virtually impossible to get a proper level of airtightness in such a retrofit and am just throwing good money after bad at the moment. If I can get the info I require on the sprayfoam suitability that might be the answer.

    Incidentally, I noticed that in one place there are 2 nails coming through the felt below a velux window. Is it feasible to trim the nails and repair the 2 holes from the inside, in order to make it watertight? They are side by side coming through for some reason(shoddy workmanship)


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  • #7
    Hidalgo
    Registered Users, Registered Users 2 Posts: 2,845 ✭✭✭Hidalgo


    sinnerboy wrote: »
    If the felt feels stiff and "tarry" and has a noticeable square grid about 10mm spacings it is not breathable.

    If it feels light and canvass like it is probably breathable.

    Some of the canvessy types that were on the market were not breathable, at least when compared to others.
    Especially if the roof was in shaded area which gets little sunlight after frost.

    Neighbour of mine built his house with the back of the roof as above with felt that was labelled breathable but has had problems every winter that there's a hard frost.originally he thought the problem was insulation touching the felt and causing dampness but that wasn't it.


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