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Flat roof insulation. To vent or not?

Discussion in 'General DIY' started by HJC1972, 26 Apr 2012.

  1. HJC1972


    14 Nov 2011
    Thanks Received:
    United Kingdom
    Posted under another section but no answers so I thought I'd re-post here.

    Hi, I am about to finish insulating the roof on my garage. It is approx. 6.5mtrs X 2.5mtrs wide with the joists running across the width. It currently has a cold deck and I have 120mm celotex to put between the rafters. The joist are notched into the bricks with the top of the joists sitting level with the top of the bricks. The ply roof substrate also sitting onto the bricks and extending over the sides.

    I am unsure if I should put the celotex tight up under the ply thus leaving a cavity beneath this and the plaster board that I will eventually dry line it with. Alternatively, should I fit the celotex sitting just above the plaster board with the cavity above. If I do this do I need to knock out a brick at each end between each andd every rafter or could I perhaps knock out a brick every second rafter and dril some vent holes, say, approx. three or four 30mm in the joists to allow the air to move down the length?

    Alternatively again, if I was to fit the insulation tight to the underside and use a vapour barrier would this suffice?

    Your help appreciated.
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