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Warm roof insulating

  • 10-06-2021 10:42pm
    #1
    Ginger83
    Registered Users Posts: 3,415 ✭✭✭


    On a warm roof build up I can only fit 100mm insulation on the deck as I'm restricted with an upstairs window.
    Can I add extra between the joists or an insulated board to the ceiling?
    Will this make it a cold roof needing vents defeating the purpose of the 100mm on top?


Comments

  • #2
    10-10-20
    Registered Users Posts: 6,641 ✭✭✭10-10-20


    Not unless you want a hybrid roof:
    https://www.youtube.com/watch?v=RthYYgzhMWU&t=103s


  • #3
    Ginger83
    Registered Users Posts: 3,415 ✭✭✭Ginger83


    10-10-20 wrote: »
    Not unless you want a hybrid roof:
    https://www.youtube.com/watch?v=RthYYgzhMWU&t=103s

    Thanks but I'll stick with the warm roof. It complies with what I can fit.


  • #4
    PMBC
    Registered Users Posts: 1,102 ✭✭✭PMBC


    Is the length of the roof causing the top end to be that close to the cill of the upstairs window? Are you using the full width of your rear garden?


  • #5
    Ginger83
    Registered Users Posts: 3,415 ✭✭✭Ginger83


    PMBC wrote: »
    Is the length of the roof causing the top end to be that close to the cill of the upstairs window? Are you using the full width of your rear garden?

    Yes the fall comes close to an upstairs cill so the roofers want to leave enough space for lead flashing.
    We went with 100mm and I got Xtratherm to calculate the u values.
    We are going with a flexitec fibreglass finish.


  • #6
    PMBC
    Registered Users Posts: 1,102 ✭✭✭PMBC


    You most likely need to insert a cavity tray under that window cill rather than just a standard flashing- a construction detail often overlooked - as the wall under the new roof and existing cill will become an internal wall and need to be protected from water/moisture in the cavity.
    If you were very intent on more insulation, perhaps you could grade the roof in the other direction, slope it to the side (thus my question on using full width) or slope to the centre form both ends. Away from existing rear wall is best, though.


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  • #7
    Ginger83
    Registered Users Posts: 3,415 ✭✭✭Ginger83


    PMBC wrote: »
    You most likely need to insert a cavity tray under that window cill rather than just a standard flashing- a construction detail often overlooked - as the wall under the new roof and existing cill will become an internal wall and need to be protected from water/moisture in the cavity.
    If you were very intent on more insulation, perhaps you could grade the roof in the other direction, slope it to the side (thus my question on using full width) or slope to the centre form both ends. Away from existing rear wall is best, though.

    Is that fitted instead of lead or with it?


  • #8
    PMBC
    Registered Users Posts: 1,102 ✭✭✭PMBC


    slightly different idea and like an enlarged/widened flashing. Google/youtube has good illustrations as in how to install a cavity tray


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