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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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    • CommentAuthorTriassic
    • CommentTimeMay 16th 2015 edited
     
    I'm building a small 7 x 5 m single storey, timber framed, home and having read up on insulation options, I decided to use Celotex between each of the studs and the rafters, prior to fitting a vapour membrane and plaster board.

    So the insulation comes in 8 x 4 foot sheets and in this case is 145mm thick, to match the stud depth. Each sheet needs to be cut up into various strips, mainly 400mm wide. So the first task was to try various cutting techniques, mark out and hand saw being the first, quite difficult to get a square edge and keep a straight line. Next I tried using a circular saw and a cutting guide, with care you cut both sides of the sheet and using a hand saw, finish off the cut. This method gives the straightener cut.

    Next to fitting the Celotex strips, I tried to cut each one so they were a tight fit between the studs, not as easy as I thought, as the studs being timber may be slightly twisted or a few mm out, so you have to fit them as best you can, with as few gaps as possible. Any gaps are then filled with expanding foam, a bit of a messy job to start with, until you gain experience of just how much foam to inject to fill the gap and no more.

    A few final observations, the Celotex has a foil covering on both sides and this wrinkles during the manufacturing process, making it impossible to get an intimate fit between the insulation board and the raking board (OSB).

    It's also a messy and time consuming process. Maybe next time I'll blow in cellulose.
    • CommentAuthorcjard
    • CommentTimeMay 16th 2015
     
    Different PIR boards behave differently. Celotex does wrinkle, I've found.. But the reticel and kingspan I've been using doesn't seem to wrinkle anywhere near as much. You don't need an intimate fit between osb and PIR because any air cavity less than about 20mm doesn't really convect so that forms an insulative layer too. further, any air cavity at all means the foil face can do its job of reflecting radiant heat, and the gap for this to work is normally 20-40mm from manufacturer recommendation

    Yes it's a bugger to fit. Personally I'd have recommended to use wool, and line the inside with 75mm of PIR. The overall cost is lower. The overall performance of the wall is better and the speed of install is higher because cutting the stuff is a pain in the arse. The trade off is the loss of 3 inches.. Make the building 3 inches bigger. I've just bought an old table saw that I'm hoping will take a 14 inch blade so I can cut a 140 sheet (score n snap) easily, but it won't ever beat wool or warmcel for speed and ease

    Next time cut it 5mm smaller all round and foam it in. Saves pounding the hell of out it with a 5x2 off cut :)

    Oh and after doing both sides with your circ saw it should just snap cleanly, no need for sawing by hand
    • CommentAuthorTriassic
    • CommentTimeMay 16th 2015 edited
     
    Someone at a trade show had suggested a mix of PUR board and cellulose, in the following build up...

    12mm plaster board
    VCM
    25mm PUR board over studs
    75mm PUR between studs
    105mm cellulose (I have 180 mm deep studs)
    18mm OSB
    Breather membrane
    Rain screen

    The idea is the thinner PUR board can be easily cut and any gaps will be filled/sealed by the cellulose. I do need to check the finished U value though.
    • CommentAuthorEd Davies
    • CommentTimeMay 16th 2015
     
    Re the trade show suggestion, which I quite like:

    · Why not put the VCM under the first 25 mm PUR, directly on the studs? Would be more convenient to install (staple direct to studs), protected a bit from service installation, and PUR over helps block any holes. Easily far enough inboard that there wouldn't be any condensation issues.

    · 18 mm OSB seems a bit much. I realise that cellulose is fairly heavy and well packed in but even so I'd have thought something a bit lighter would be plenty for a single storey house, particularly with 400 mm c/c studs.

    Going back to the first post: if fitting PUR between studs against OSB I wonder if it would be a good idea to put thin strips of wood (10 mm or less, perhaps 20 mm wide strips of 9 mm ply) horizontally at intervals of something like 600 mm vertically to deliberately hold most of the PUR off the OSB so that the foil works but has a good seal at intervals vertically to prevent significant convection.
    • CommentAuthorborpin
    • CommentTimeMay 17th 2015
     
    There is no good way to do it.

    Fundamentally this is the wrong way to build. I'm with VH on this. TF and EPS on the outside rendered. Yes I know not everyone can do it but I am convinced it is the right way to go, EPS underneath, and a flat roof with EPS on top - a complete teacosy. Run services in the TF and in metal web joists. No membranes, no tapes - simple construction.

    This country's planners and builders are so stuck in the dark ages and need to see the light instead of following post war building methods.

    All I need now is to finish this one (not done this way) and start the next. :)
    • CommentAuthorborpin
    • CommentTimeMay 17th 2015
     
    Posted By: TriassicIt's also a messy and time consuming process. Maybe next time I'll blow in cellulose.
    Unless you have *really* deep studs it is difficult to get the same U values. EPS on the outside gets my vote.
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