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Vapour control barrier and Ventilation

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  • 10-01-2006 3:47pm
    #1
    awishawash
    Closed Accounts Posts: 135 ✭✭


    I have read up on this vapour layer thing as much as possible and I cant understand it. I am putting vapour barrier sheeting under my ceiling insulation at the moment(flat roof), and there doesnt seem to be anywhere for the air to escape. Should I put a vent in the flat roof? Also when the moisture from hot air is going out through the plasterboard wont it stop at the plastic sheeting and end up dampening the plaster? Is this idiotic.


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    #2
    patrido
    Closed Accounts Posts: 299 ✭✭patrido


    the moisture that is present within the warm air of the house only gets damp (condenses into a liquid) when it cools down as far as the dew point.

    as it is on the warm side of the insulation, and is prevented by the polythene from leaving the thermal envelope of the dwelling, it will never reach that point.

    if it was not prevented from leaving, and was allowed to enter the insulation layer, it would at some stage, reach the dew point and turn into a liquid. this might happen above the insulation layer in which case the ventilation in your roof space (if it's good enough) would have to take care of it.

    or it might condense within the insulation layer. the consequenses of this depend on the type of insulation, and range from thermal bridging (reducing the effectiveness of the insulation) to rotting the insulation and/or the roof timber.


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    #3
    Victor
    Registered Users Posts: 78,352 ✭✭✭✭Victor


    awishawash wrote:
    I am putting vapour barrier sheeting under my ceiling insulation at the moment(flat roof), and there doesnt seem to be anywhere for the air to escape.
    You need to put vents in the eaves / parapet and possibly elsewhere (to allow a crossflow). I'm sure Pete can suggest a detail.

    Does the flat roof cover the entire building or does it meet higher walls / roof?


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    #4
    awishawash
    Closed Accounts Posts: 135 ✭✭awishawash


    Yes its a flat roof over an old garage. Not sure where to put a vent.


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    #5
    rooferPete
    Registered Users Posts: 1,497 ✭✭✭rooferPete


    Hi,

    A lot depends on the waterproofing membrane you plan to use, most of the modern membranes be they single ply or built up systems can be vented at the upstands or at the drip detail.

    It is also possible to fit pressure relief vents in the roof build up that will allow the roof to vent above the build up or you could consider a "warm roof" where the insulation is fixed above the deck.

    Unless you are using a skilled roofer you can have the roof terminating at the gable wall of the house where you can allow the roof to vent behind the cover flashing.

    I'm not sure if the above is of any help to you because there are many different formats of garage roofs.

    .


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