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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,

    Adam
      SECTION-2.jpg
    •  
      CommentAuthordjh
    • 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
     
    DJH,

    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,

    Adam
      SECTION-22.jpg
    • 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,

    Adam
      WINDOW_DETAIL2.jpg
    • 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,

    Adam
    • 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
     
    Daryl,
    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!

    Adam
    • CommentAuthorDarylP
    • CommentTimeJan 25th 2018
     
    CRA - Condensation Risk Analysis
    I would charge you as a forum member £35.00 all inc.
    KR,
    Daryl
    • 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.
    • CommentAuthoradam_w
    • CommentTimeJul 17th 2018
     
    Morning All,

    I'm now in the midst of my extension and currently at the EWI stage. I think the most effective and simplest window mounting solution is the ply box version but how far out should the window sit i.e. dimension 'x' in the drawing below;

    Thanks in advance,

    Adam
      Window_box.jpg
    •  
      CommentAuthordjh
    • CommentTimeJul 17th 2018
     
    To some extent it's a matter of choice based on waterproofing concerns and appearance etc.

    From a thermal point of view what's essential is that the line of the glass is no further inboard than the joint between insulation and internal mass, but I guess your timber frame is insulated as well. Thermally optimum is around the middle of the overall insulation.

    From a waterproofing point of view, having it further out means less area to protect at the bottom especially corners, but more to protect above the head of the window. Further in vie-versa.

    It will also decide how deep your internal window boards are and what depth of external sills you need.
    • CommentAuthoradam_w
    • CommentTimeJul 17th 2018
     
    Thanks Dave,

    As you can see from the diagram, my intentions are to push the window right out in to the insulating layer. I've checked with an online window supplier and they offer an 85mm sill so I would probably look to set the window 70mm back from the most outer face to allow for decent amount of weathering.

    Cedral, like most cladding suppliers recommend a breathable membrane between the EWI and the battening (shown in green) but would it also be worth adding some flashing (shown in red) around the protruding box? Possibly even tipping the box forward at the top to create a drain off?

    Thanks again,

    Adam
      Window_box_2.jpg
    •  
      CommentAuthordjh
    • CommentTimeJul 17th 2018
     
    Yes, definitely add flashing; I would carry it to the end of your ply but you'd want to arrange that any water that got there was dripped onto the outside of the window (or directly onto the sill) rather than be directed inwards by the green membrane. I suspect the green membrane should just finish at the bottom of the vertical piece and not be turned inwards. Generally waterproof components need to move outwards through the construction as they move downwards.

    You could tilt the box, although I suspect that would be awkward to do. Simpler might be a triangular cross-section of inssulation or timber on top of the ply for the red membrane to rest upon. But depending on the exact geometry, it may be overkill.
    • CommentAuthoradam_w
    • CommentTimeJul 18th 2018
     
    Morning Dave,

    Thanks again for your valued input, the ply is on order and I will be incorporating all your suggestions this weekend! I think with regards to including an angle above the box, timber would probably be the easiest way to go, might be a little tricky but I think it'll we worth it just for piece of mind.

    Oh and P.S. check out the photo for progress :D

    Thanks again,

    Adam
      17-7-2018.jpg
    •  
      CommentAuthordjh
    • CommentTimeJul 18th 2018
     
    Looks good. What are the brown squares?
    • CommentAuthoradam_w
    • CommentTimeJul 18th 2018
     
    Thanks, they are OSB pads that I screwed in to the studs to hold the EPS in place while it dried, its been pretty windy the last couple of days down here!
    • CommentAuthoradam_w
    • CommentTimeJul 19th 2018
     
    Morning again and another question...

    Would you recommend flashing under the window or because there is nowhere for water to sit, would you deem it unnecessary?

    Thanks again,

    Adam
      WINDOW DETAIL 2.jpg
    •  
      CommentAuthordjh
    • CommentTimeJul 19th 2018
     
    I would put the flashing in and I would extend it out as far as the drainage cavity. In my case I also extended it under the window board inside, but I'm paranoid about water getting to the top of my bales.
    • CommentAuthoradam_w
    • CommentTimeJul 19th 2018
     
    Thanks Dave, any product recommendations? Externally for OSB to OSB I've been using Bostik Flashband but its quite thick.

    Thanks,

    Adam
    •  
      CommentAuthordjh
    • CommentTimeJul 19th 2018
     
    I don't remember exactly what I used. I think it was Pro Clima membrane and I think it was blue, which would make it DB+ I think. But thinking about it now, even something like DPM sheet should work I think?
    • CommentAuthoradam_w
    • CommentTimeJul 19th 2018
     
    Dave, without the adhesive backing, would the DPM not crinkle or shift during the build?

    My next question is about the frames themselves, are they screwed from inside the frame out to the box or from outside the box in to the frame?

    Cheers,

    Adam
    •  
      CommentAuthordjh
    • CommentTimeJul 20th 2018
     
    Posted By: adam_wDave, without the adhesive backing, would the DPM not crinkle or shift during the build?

    I don't think it matters if it does crinkle a bit, but you could always use some adhesive to hold it in place.

    My next question is about the frames themselves, are they screwed from inside the frame out to the box or from outside the box in to the frame?

    IMHO, it's a lot easier to screw from outside the box into the window frames. And then from inside the box into the sub-frames in the wall.
    • CommentAuthoradam_w
    • CommentTimeJul 20th 2018 edited
     
    My next question is about the frames themselves, are they screwed from inside the frame out to the box or from outside the box in to the frame?

    IMHO, it's a lot easier to screw from outside the box into the window frames. And then from inside the box into the sub-frames in the wall.


    My only concern with this would be if I needed to replace the window at any point in the future, I wouldn't be able to access the screws that hold the window in the box, however on the other hand, I'm not sure 18mm of ply with provide enough grip for the screws from the inside-out either, what a dichotomy!
  1.  
    Posted By: adam_wI'm not sure 18mm of ply with provide enough grip for the screws from the inside-out either, what a dichotomy!

    So just use more screws!
    And drill the frame first.
    • CommentAuthoradam_w
    • CommentTimeJul 20th 2018
     
    Posted By: Peter_in_Hungary
    Posted By: adam_wI'm not sure 18mm of ply with provide enough grip for the screws from the inside-out either, what a dichotomy!

    So just use more screws!
    And drill the frame first.


    Hmmm, seems a bit too simple Peter, surely I should use a custom made gold plated bracket instead :wink:

    Seriously though, you're right, thanks for the suggestion, it sounds good enough for me :bigsmile:
    •  
      CommentAuthordjh
    • CommentTimeJul 20th 2018
     
    You then have a bunch of holes in your frame that you need to fill. I rather preferred to keep my shiny new windows in pristine condition. :cool:

    My plan if and when I need to replace a frame is to unscrew the box from the sub-frames and then take out the box and window together. Yes, there'll be a mess but there'll be a mess anyway and it's hopefully a rare event.
   
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