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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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General: PIR / PUR / Breather Membrane/ Cavity Advise

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  1.  
    • CommentAuthormoogaloo
    • CommentTimeMay 7th 2007
     
    I have been looking at the ecotherm, kingspan, celetex rigid board insulation for a pitched roof. trying not to loose too much height?

    I have 1930s 3 bed semi with a pitched roof tiled, felt and battened (do not want to reroof) and 100mm rafters. I think it will be a between and under rafter style and will need some kind of cavity and perhaps a breather membrane. But I don't actually know what I am talking about.

    How much cavity should I have? Should I have breather membrane? Should I consider multifoil in the mix (don't hit me)? Are the required U value of 0.2 against the U Value of 0.16 of joist insulation for legitimate reasons or should I be shooting beyond 0.2?

    Help me please :cry:
  2.  
    I think you need some help. You are bouncing up against possibly the most difficult detail in contemporary building, how to insulate a pitched roof. Traditional theory holds that you must maintain a 50mm vent gap between insulation and underfelt, which halves your available 100mm rafter depth. Whether this is strictly necessary is open to doubt but no self respecting building inspector would let you get away without a vent gap unless you have a breather felt above it. And you can't fit a breather felt under an already non-breather felt: well you can, but it wouldn't do any breathing so it would be pointless. Which leaves you having to insulate under the rafters as well, which eats up your headroom. It's a real headache.

    Why doesn't someone invent a really neat silver foil type of insulation that does the work of 200mm of mineral wool in, say, 25mm. That would be a really neat solution.:swingin:
    • CommentAuthormoogaloo
    • CommentTimeMay 8th 2007
     
    I know the multifoil folks seem to drastically over estimate there effectiveness. But what about this

    40mm Air Gap
    50mm of PIR Board
    30mm of Multifoil staples and overlapped with 20mm Cross Battons
    Plasterboard

    Which should mean I only lose about 30mm. 30mm Supreme Multi 5 "claims" a U Value of 0.2 on it's own from an a hot box test, but with the multifoil controversy If I can put an additional 50mm layer of PIR board, I must be in the ball park?
  4.  
    HI Moogaloo,

    Have a look at the web dynamics BBA whick covers the method you are proposing

    http://www.bbacerts.co.uk/certs/43/4379i1_web.pdf

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