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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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    • CommentAuthoradam_w
    • CommentTimeDec 6th 2017
    Good morning All,

    I've recently been granted planning permission to add a 2 storey extension to the side of my detached 3 bedroom brick and block work house down on the south coast. The house currently has a rather sorry looking garage via a shoddy corridor to the side which I plan to knock down and rebuild. The plans are for the ground floor to stay as a garage and the top half to be a 4th bedroom and en-suite. I've attached a drawing of my intended 'build-up' of the walls hoping that the experienced and well informed ones amongst you will be able to offer me some valued words of advice. The bottom half of the house is to be rendered in Weber Pral M and the top half clad in Cedral weatherboard. I've based my current design on the 'warm frame' principal where all habitable spaces are externally insulated with additional 'inter-stud' insulation to help meeting building regs. Any information or recommendations you could offer would be greatly appreciated!

    Thanks in advance,

    • CommentTimeDec 6th 2017
    Where's the mebrane under the tiles?

    The joists form a big thermal bridge.

    How is the floor insulated?

    How is the roof insulated?

    Where's the fire separation from the garage to the room above?

    How are the fascia, soffit and gutter attached?

    How are the counterbattens attached?

    Why not use thicker insulation on the outside of the frame?

    Where's the DPM?

    Where's the DPC under the studwork?

    What's under the sole plate?
    • CommentAuthoradam_w
    • CommentTimeDec 6th 2017
    Mr DJH, thank you for your questions, the points you've made are exactly why I've chosen to join this forum. Please note that I am not a builder or any type of related tradesman, just a keen DIY'er looking to learn.
    Anyway, my answers are as follows;

    Q. Where's the mebrane under the tiles?
    A. Missing from the drawing, i'll shall amend to suit.

    Q. The joists form a big thermal bridge.
    A. I was aware of this but I thought my 60mm PIR would be sufficient to dampen this effect?

    Q. How is the floor insulated?
    A. Again, this is missing from the drawing but I was planning on some 200mm of Earthwool style in between the joists.

    Q. How is the roof insulated?
    A. This detail is also missing from the drawing but as I was hoping to have a vaulted ceiling to compensate for the lack of head height at the eaves, I was going to fit 100mm PIR inter-rafter.

    Q. Where's the fire separation from the garage to the room above?
    A. Again, missing from the drawing but I was planning on using fireboard (doubled if required) sandwiching some 25mm PIR for added insulation

    Q. How are the fascia, soffit and gutter attached?
    A. I am struggling with this detail but I have put a Trapazoid in the model that I was envisaging pinning through the PIR in to the rafters at 400mm centres but I think that may be problematic, looking at it now, it may be better to run a 4x2 and then pin a smaller trapezoid to that if that makes sense?

    Q. How are the counterbattens attached?
    A. I assumed the vertical battens would be pinned straight into the rafters sandwiching the PIR and then the horizontal ones would be pinned in to those.

    Q. Why not use thicker insulation on the outside of the frame?
    A. I'm trying to limit wall thickness and keep insulation thickness common throughout the design to reduce cost.

    Q. Where's the DPM?
    A. I wasn't sure how important this would be to my build as its only intended to be used as a garage?

    Q. Where's the DPC under the studwork?
    A. Unintentionally missing from the drawing :)

    Q. What's under the sole plate?
    A. I was planning on a double course of brickwork, maybe engineering bricks, or could I get away with just extending the oversite to the outer sub DPC wall?
    • CommentAuthoradam_w
    • CommentTimeDec 7th 2017

    I've taken on pretty much all of your comments and amended my drawing to suit, thanks again for your opinions, they are greatly valued, hopefully I'm a lot closer to my final build-up this time round;

    Kind regards,

    • CommentAuthoradam_w
    • CommentTimeDec 20th 2017
    Would anyone be able to offer me some advice with regards to window location on the 1st floor using the above construction method please? Should it sit in the middle of the timber frame, leaving me needing rather deep (70mm) sill or can I push them further out and only rest the inner lip of the window frame on the timber aperture? Thanks in advance, Adam
    • CommentAuthortony
    • CommentTimeDec 20th 2017
    Yes, push the window as far out as you can overlap the frame with insulation on the outside by say 20mm if there is room, chamfered if you like
    • CommentAuthoradam_w
    • CommentTimeDec 21st 2017
    Hi Tony,

    thanks for your reply, I'm slowly getting there with the details now but just to clarify, could you confirm that the situation I've drawn below is as per your recommendation?

    Many thanks,

    • CommentAuthoradam_w
    • CommentTimeJan 17th 2018
    Good morning everyone, I'm still developing my build-up as shown above and its been highlighted to me that the PIR on the external face could cause moisture issues as its not breathable. With this in mind i'll swap that out for some Pavatherm plus, only downside to that is its U value is half of that of the PIR board so i'll need to compensate for that elsewhere. Although tradition states that timber walls are built us using CLS 89x34mm timber, would there be any reason why I couldn't use 75x45 SCANT as an alternative? This would allow me to use those extra 14mm for internal PIR board, further reducing my thermal bridging. I'm meeting with building control next Friday so any input would be greatly appreciated.

    Thanks in advance,

    • CommentAuthorgravelld
    • CommentTimeJan 17th 2018
    See earlier comments about making the external insulation thicker? ;-)
    • CommentAuthoradam_w
    • CommentTimeJan 17th 2018
    Thanks Gravelld, so you think scrap the Internal PIR board and add an extra 20mm to the Pavatherm even though it's twice the conductivity?
    • CommentAuthoradam_w
    • CommentTimeJan 18th 2018
    I'm happy with the inter-stud PIR but my concerns were that the continuous outer PIR board would prevent any moisture from the frame itself escaping but I am very new to all of this and have only really learnt what I've read on this forum. If that isn't the case and my initial detailing is correct then I'm more than happy as, as I've previously mentioned, PIR is a lot more thermally efficient and cheaper than the breathable equivalent!

    Thanks again for your comments, I really do value and appreciate them!

    • CommentAuthorDarylP
    • CommentTimeJan 25th 2018
    CRA - Condensation Risk Analysis
    I would charge you as a forum member £35.00 all inc.
    • CommentAuthorgravelld
    • CommentTimeJan 25th 2018
    Unless it was London or someone with high real estate prices I'd be avoiding PIR totally and redesigning with extra depth of insulation. But it depends on your goals.
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