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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.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthormitchino
    • CommentTimeMay 20th 2020
     
    I am costing up having our shabby and leaky, double low pitch (12 degree), felt covered, cold roof stripped and redone as a warm roof. Plan would be to insulate between and over new rafters. I don't have a lot of room to play with to maintain the same ridge height. Building regs I think require a U-Value of 0.18W/M2K. I looked at the Kingspan online u-value calculator and it's saying I can achieve this with 60mm between the rafters and 60mm above. Is that right?
  1.  
    Just had a look out of interest and that is what it seems to say. If you increase it to 100mm between and 100mm over it gets you to 0.11
    Is there any reason you can’t increase the ridge height by the additional 40mm?
    Do you know what size rafters you will have? If they are 125mm deep then 100 will fit and still leave a gap for cables under the plasterboard!
    We did this and fixed through counterbattens with twist fixings.
  2.  
    Will I sound pedantic if I say that anything between plus something above is not a pure Warm Roof, per se? It's a hybrid Warm roof, and IIRC the rule of thumb is 2/3 of the R outside and 1/3 under - ish.
  3.  
    I thought you were okay as long as you have more above the rafters than between (or at least the same).
    I suppose technically it’s a hybrid roof but then I guess some people’s roofs are warmer than others!
  4.  
    That Kingspan calculator referred to in the OP seems to automatically split it 50/50 between and over.
    •  
      CommentAuthordjh
    • CommentTimeMay 21st 2020
     
    It will also depend on the rafter spacing. With insulation between as well as over, you need to run a condensation analysis.
    • CommentAuthormitchino
    • CommentTime7 days ago
     
    Thanks folks, I've decided to shelve the plans for the warm roof for now and just refurbish the existing cold roof, I plan to have 100mm PUR between ceiling joists and then another 100mm PUR or 200mm quilt cross laid over the joists.
    • CommentAuthormitchino
    • CommentTime5 days ago
     
    I've had an idea regarding my cold roof refurbishment. A few years back I crawled into the eaves on my belly (remember the roof pitch is 12 degrees!) and installed Klober vents, so air would flow up and over my rolled out insulation, and I could stuff it into the eaves without blocking the ventilation. Now I'm thinking of installing PUR, I was wondering whether I could cut the PUR so it slides snugly into the eaves, and also cut out ventilation channels in the top of it to do the job of the Klober vents, and get rid of them. Would this work?
    • CommentAuthormitchino
    • CommentTime2 days ago
     
    anyone got an opinion on my vented PUR idea?
    • CommentAuthortony
    • CommentTime2 days ago
     
    I would do that yes and yes but also not snugly but with a 10mm gap 🙂
    • CommentAuthormitchino
    • CommentTime2 days ago
     
    Thanks Tony - so keep a 10mm gap between roofing sheets and sloping top of PUR, plus 25mm square channels cut out of the PUR where it meets the slope of the roofing sheets?
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