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spray foam insulation / ventilation cards

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  • 20-10-2014 7:57am
    #1
    Pedaaa
    Registered Users Posts: 3 ✭


    Hello,
    I found some interesting threads here about spray foam.
    I am living in Austria, where this type of insulation is pretty rare.

    But i am going to use spray foam insulation for my attic.
    Its an 1970s house, with a bitumen felt layer below the tiles.
    We have one Icynene spray foam company here in Austria, but they usually dont use any vent cards. (maybe because newer roofs usually use breathable felts anyhow)

    But as i have a non-breathable bitumen felt installed i´d prefer to install a breathable vent card before spraying over it.
    By doing this, I think that vapor can go through the foam and the vent card, but will condensate at the bitumen felt.
    But because of the ventilation gap, this will be no problem, correct?!

    please google pictures for "breathable vent cpi spray foam"
    (as i cannot post pictures as a new user)
    This is how i would like to do it.

    I am now searching for such breathable vent cards.
    We don´t have any suppliers for this here in Austria.

    But for me, it looks like a simple perforated plate anyhow.

    So...maybe I can build my own vent-cards?
    Can anybody tell me specifications needed for this perforated plate?
    (i.e. hole size and quantity? small enough that the foam will not expand through it, but also big enough to be breathable)

    Hope you can help!
    many thanks!!

    greetings,
    Peter


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    #2
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,140 Mod ✭✭✭✭BryanF


    Corriboard make a good vent card. Have you ventilarion at the eaves and ridge that will line up with the vent cards? I'm not sure about spray foam companies breathability claims..


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    #3
    Pedaaa
    Registered Users Posts: 3 ✭Pedaaa


    Hi,

    at the eaves there is a small gap. I can see to the outside there. I would rework this a bit and fit the vent cards there, to get a nice transition. So problem on this side.
    On the upper side: I have no vented ridge, but i am going to install some vented tiles.

    Do you really think that Corriboards let vapor go through???
    Any experiance with spray foam with that?

    thanks!


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    #4
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,140 Mod ✭✭✭✭BryanF


    Pedaaa wrote: »
    Hi,

    at the eaves there is a small gap. I can see to the outside there. I would rework this a bit and fit the vent cards there, to get a nice transition. So problem on this side.
    On the upper side: I have no vented ridge, but i am going to install some vented tiles.

    Do you really think that Corriboards let vapor go through???
    Any experiance with spray foam with that?

    thanks!
    Is the foam breathable?

    You will be taping & sealing a vapour barrier on the inside?. Joists are never perfect so the u-shaped corriboard will let residual moisture into the vent zone and assuming the cavity is vented then what's the concern? Why are you so keen on foam, over the more traditional insulation in Austria?


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    #5
    Macspower
    Registered Users Posts: 3,191 ✭✭✭Macspower


    I would have a concern about the use of non perforated coriboard as a "vent" card.

    It would just transfer the dew point from the felt to the surface of the coriboard?


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    #6
    Pedaaa
    Registered Users Posts: 3 ✭Pedaaa


    Hello, i am now doing it in a different way:
    I won't use vent cards or channels.
    I will install a variable vapor retarder inside.
    There are such types available, that does not allow diffusion from inside to outside in winter.
    But it does allow diffusion back to the inside in summer.

    So...
    Vapor can't enter the insulation.
    And even if some small amount of water would enter the insulation, it can go back to the inside an dry.

    I guess this will be the best solution for my situation, with the bitumen-felt as upper layer


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    #7
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,140 Mod ✭✭✭✭BryanF


    Pedaaa wrote: »
    Hello, i am now doing it in a different way:
    I won't use vent cards or channels.
    I will install a variable vapor retarder inside.
    There are such types available, that does not allow diffusion from inside to outside in winter.
    But it does allow diffusion back to the inside in summer.

    So...
    Vapor can't enter the insulation.
    And even if some small amount of water would enter the insulation, it can go back to the inside an dry.

    I guess this will be the best solution for my situation, with the bitumen-felt as upper layer
    You guess Vapour can't enter? What if does, do the roof timbers have to wait until summer for moisture to make its way back into your house to allow them to dry out? How does the vapour know it's summer?

    Roof timbers need ventilation.

    Building regs and all advice going back donkeys years requires ventilation between insulation and non breathable felt.
    http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,18751,en.pdf refer to p12,14,16


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    #8
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,140 Mod ✭✭✭✭BryanF


    Macspower wrote: »
    I would have a concern about the use of non perforated coriboard as a "vent" card.

    It would just transfer the dew point from the felt to the surface of the coriboard?

    Punch some holes in the corrie board so?


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