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Insulating a flat roof

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  • 13-08-2009 8:45pm
    #1
    genie
    Registered Users Posts: 975 ✭✭✭


    I have an extension built in the 1960s at the back of my house with a block cavity wall and a flat roof. I want to get cavity wall insulation and I'm wondering if the same insulation can be pumped into the space between the ceilings and the flat roof?

    Many thanks! :)


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    #2
    Martron
    Registered Users Posts: 2,930 ✭✭✭Martron


    i dont see why not.

    would the ceiling be able to support the weight of the insulation? you will need to put holes in your ceiling to pump the insulation in.


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    #3
    PeteHeat
    Registered Users Posts: 648 ✭✭✭PeteHeat


    Hi,

    Are you sure its not 9" cavity block ?

    As that would have been fairly standard for extensions in the 60's / 70's.

    Good insulation companies will tell you its a waste of money pumping insulation into a 9" cavity block.

    Also take care with pumping insulation into the roof space it will fill between the joists preventing air flow between the insulation and roof deck causing the deck to rot from underneath.

    .


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    #4
    irelandhouse
    Registered Users Posts: 139 ✭✭irelandhouse


    Pete, why do you reckon its not recommended ? not needed ?

    . what is the make up of the roof?
    - 5 or 6x2 ? do you have access to the cavity? i.e. is the ceiling inside down ?


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    #5
    Carlow52
    Closed Accounts Posts: 2,389 ✭✭✭Carlow52


    irelandhouse wrote: »
    Pete, why do you reckon its not recommended ? not needed ?

    . what is the make up of the roof?
    - 5 or 6x2 ? do you have access to the cavity? i.e. is the ceiling inside down ?

    I think PH means that it is virtually impossible to pump a 9" cavity block wall which is not a cavity wall..
    On the flat roof being early 60's its well due to be redone so before doing any insulation I would consider redoing it: best plan is to go slated if u have the head room for the 20 degree slope


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    #6
    PeteHeat
    Registered Users Posts: 648 ✭✭✭PeteHeat


    Hi,

    Thanks Carlow,

    Even if you could be sure that each of the holes in the cavity blocks were filled there is still the problem of the cavity being bridged at every course, if anything blocking the cavity could cause damp problems (thermal bridging).

    Filling the roof space between the joists with insulation has a similar result called interstitial condensation basically the heat from the ceiling side of the insulation meets the cold from the roof causing condesation within the insulation.

    Combine the above with no air flow under the roof timbers and you get rotting timber in a very short time, this is even worse in kitchens and bathrooms due to the amount of heat and moisture laden air.

    Most buildings of that era did not have vapour checks or foil back plaster boards to prevent the moisture from entering the roof space.

    Definitely insulate the room, dry lining the walls can work wonders, there are different ways to insulate the roof, take the ceiling down and do it properly or use an upside down insulation on the roof.

    .


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