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Closing top of cavity on new build

  • 23-01-2023 10:27am
    Registered Users Posts: 156 ✭✭

    Hi, Just looking for some suggestions on the options for closing the top of the cavity in a new build - the walls are being pumped (250mm cavity). Looking at the roofing drawings it does look like the engineer is suggesting using 25mm rigid insulation resting on the inner and outer leaf wall plates (in-between the rafters I would assume with the rafter "closing" the cavity at locations where they are sitting). My blocklayer thought it sounded an awkward and expensive way to do it and he suggested maybe using DPC to close it and then add Rockwool or similar on top of the DPC to insulate.

    I was wondering myself if jamming rigid insulation cut to fit into the cavity similar to how the cavity has been closed at windows with some spray foam to hold/close any small gaps would be a better way but I would need to do it before rafters go on (which adds time pressure for me as only have weekends really free for hands on work). It's also lot of cutting and hard to size right. Also nothing is suggested for the top of gable walls but it would seem these would have to be "stuffed" rather than have anything laying across the leaf.

    I will of course be discussing with my engineer but I always like to hear some opinions on it before hand.


  • Registered Users Posts: 8,114 ✭✭✭Furze99

    This thread might be useful

    Ourselves - used a block laid flat to top the cavity wall and the wallplate rests on that.

  • Registered Users Posts: 156 ✭✭SodiumCooled

    Thanks for pointing out the other thread. We went with the DPC option in the end. Its was put under the inner and outer wall plates so when nailed in place hold it down very securely and close the cavity completely to prevent any pumped insulation escaping. We can add further insulation on top of the DPC if continuous insulation is needed up to the rafters.

    As an aside, using a block to close a cavity is no longer done due to thermal bridging even though it was very common at one time.

  • Registered Users Posts: 1,691 ✭✭✭fclauson

    the closure should be fire proof as well as not being a thermal bridge