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  1.  
    I have 150mm PIR between the rafters

    I want to put some insulation under the rafters.

    **However useable head height is important as most of the bedroom roof is sloping.**

    BTW BCO says we do not have to to put any more insulation under the rafters to get it passed

    A problem is that we have wooden bracing 25mm deep going across the rafters in diagonals at certain random places to keep the roof trusses secure. Building control say we could remove the braces if we use 9mm of ply sheeting thorughout the roof.

    That sounds expensive especially with labour

    It was my provisional plan to put PIR plasterboard with 25mm insulation. We would cut the insulation out where the bracing is.

    Another method might be to put battens across the rafters/joists which would be the same depth as the bracing and infill with plain PIR board and screw ordinary plasterboard onto battens.

    Which do you think is best / cheapest. Any other ideas ? Please remember ceiling height is critical

    Thanks
    • CommentAuthorMarkyP
    • CommentTimeJan 29th 2018
     
    See if BC will agree to you replacing the timber bracing with metal flat strapping. I think you can get this in at least 1800mm lengths, maybe longer. These could be set just below the surface of the rafter with a little notching out with the chisel or a swipe or two with the plane so wouldn't cost you any ceiling height.
  2.  
    Thanks for the very good suggestion.

    We aren't allowed to cut notches into the trusses because of the span, but the reduction in height could still be useful.
  3.  
    Posted By: kayseraseraBuilding control say we could remove the braces if we use 9mm of ply sheeting thorughout the roof.
    That sounds expensive especially with labour

    OSB should be about half the price of ply !
    • CommentAuthorMarkyP
    • CommentTimeJan 29th 2018 edited
     
    metal strapping is routinely recessed below the surface of joists and rafters in my experience. The amount of joist material removed to allow this is tiny, 5mm or so. I think you'd be unlucky to find a BC who pulled out the notching guidelines for such a trivial amount of material. Different discussion if you were talking about notching out for the timber bracing which would be significantly deeper and wider than a chip out for a bit of strap. Personally I would just check they would accept strapping as a brace material, and not even ask about recessing as I think it reasonable to assume it as standard to do so.
  4.  
    Yes I will ask
    • CommentAuthorgyrogear
    • CommentTimeJan 29th 2018
     
    Posted By: kayseraseraWe aren't allowed to cut notches into the trusses because of the span,


    could you use timber connectors - that's how my purlins are fixed btw my trusses...

    gg
  5.  
    I dont think so; I think the bracing has to be a a single piece that has got to connect several trusses/rafters together on a diagonal
    • CommentAuthorbxman
    • CommentTimeJan 31st 2018
     
    If you can not convince your BCO can you put the strapping on and pack the rafters up or down so you can fit your insulated plaster flat across the two no a great deal more work than notching in the first place honour saved all round
    • CommentAuthorrevor
    • CommentTimeJan 31st 2018
     
    How much depth can you tolerate? There is a law of diminishing returns there will not be much advantage in extra insulations over what you have already got the cost benefit will not be there given the amount of work needed. One could justify extra insulation across the rafters to avoid the cold bridge through the timbers and seal the inevitable air leaks between the pir and the rafter sides which could turn out to be quite a lot given time and effect of sun on the roof. Have you considered the multifoil insulation this could give you the added insulation you require and form an air seal at the same time although considered expensive the labour cost will be smaller. I personally would not mess about with the roof structure given what you have has been designed and passed as it is. I think the solution of insulated plasterboard rebated for the bracing is actually on balance the best solution and the one I would go with.
    • CommentAuthorkayserasera
    • CommentTimeFeb 1st 2018 edited
     
    Thanks for everyone's helpful contributions

    revor; no more than 37.5mm including plasterboard under the rafters.
    I had read somethings about multifoil product and it seem the opinion was the insulation value claimed was not robust in practice.
    Thanks for giving me your opinion which method you would go for. ie. use insulated plasterboard rather than battening, PIR infill and plain plasterboard
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