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External insulation with flat roof extension

  • 14-09-2009 3:58pm
    #1
    BenDave
    Closed Accounts Posts: 6 ✭


    Hi,

    I am considering using external insulation on a house which has a flat roof extension (kitchen). As I do not have the finances to insulate the extension from the outside (it will be insulated with a slab from the inside) there will be a cold bridge between where the external insulation stops (at the bottom the 1st floor) and where the flat roof starts. Does this defeat the purpose of external insulation in this case ? Are there other alternatives ?

    THanks,
    Ben


Comments

  • #2
    glencoe
    Registered Users, Registered Users 2 Posts: 30 ✭glencoe


    BenDave

    Part of the answer to your question is what type of flat roof construction you have.
    The three basic types are shown at the link below.

    http://www.woodspec.ie/iopen24/pub/detail_b_4.1.123.jpg

    My advice would be
    a. Where the roof meets the main wall, would be to take off the existing flashing, run the new wall insulation down to the roof deck or better still to join with the existing roof insulation and then to reseal the detail.
    b. Assuming you have a cold deck (insulation between the joists) you will need to open the ceiling and make sure the insulation is packed around the perimiter as best as you can, taking care to maintain the ventilation void and the vapour barrier

    glencoe


  • #3
    BenDave
    Closed Accounts Posts: 6 ✭BenDave


    Glencoe, thank you for this, it all makes sense now ,,

    Ben


  • #4
    BenDave
    Closed Accounts Posts: 6 ✭BenDave


    Hi Glencoe,

    I investigated this further and the situation appears more complex to me now.
    If I want to get the external insulation down to reach the flat roof, I will need to take the flashing out as you pointed out. This means taking out all the plaster which covers the flashing so the flashing can be removed altogether and then applying more plaster.
    Then, the external insulation has to be sealed with the flat roof where it meets the flat roof to make it rain proof, is this current practice and easily achieved ? Is this not dangerous as in this join lies all the soundness of the construction?
    I can think of a much simpler and cheaper option, although not as good. By lifting the existing flashing it would be possible to blow insulation where the external wall meets the flat roof. This would provide a lesser degree of insulation but would be far cheaper and less risky as the flashing remains in position.

    Tanks,
    Ben


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