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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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    • CommentAuthorgravelld
    • CommentTimeAug 2nd 2018
     
    Say you were bricking up a window, or making a window smaller in a cavity wall construction.

    I'm wondering whether the simple approach of mimicking the current construction is the best way.

    What about only bricking up the internal leaf, and having the cavity and outer leaf replaced by some insulation?

    I'm assuming there's an existing lintel over external leaf of course.

    It might make for improving the installation performance of replacement windows (as well as the wall itself of course).
  1.  
    I would have no problem with that except consideration would have to be given to matching the exterior finish.
    • CommentAuthortony
    • CommentTimeAug 3rd 2018
     
    The lintel and revels are lovely to be thermal bridges

    There may not necessarily be a lintel in the outer skin.

    I would do some breaking out, remove the reveals, rebuild the inner skin toothed in, insulate the cavity same as existing, then you can do what you like with outside skin.
    • CommentAuthorgravelld
    • CommentTimeAug 3rd 2018
     
    Yeah, I wondered about the finish. It's rendered, but I wondered if having a very different material (e.g. EPS) would make cracks more likely to form?

    Tony what does "toothed in" mean?
    • CommentAuthortony
    • CommentTimeAug 3rd 2018
     
    Inner skin blockwork not with a straight join to the existing wall but part blocks cut out and new ones boned in as if it was one wall rather than a wall with a square panel of blockwork in it, best also to use the Sam blocks as original builders did.
  2.  
    Posted By: gravelldYeah, I wondered about the finish. It's rendered, but I wondered if having a very different material (e.g. EPS) would make cracks more likely to form?

    If you cover the join with glass mesh as used in EWI and feather in the render I would expect it to be OK. I would expect more of a problem matching the colour and finish between the new and old. If the wall is not too big the usual recommendation would be to re-render or re-paint the whole wall to make the change invisible.
    • CommentAuthorgravelld
    • CommentTimeAug 3rd 2018
     
    Whole wall to be repainted. By finish do you mean granularity of the render?
  3.  
    Posted By: gravelldBy finish do you mean granularity of the render?

    Yes
    • CommentAuthorgravelld
    • CommentTimeOct 4th 2018 edited
     
    Another question on this.

    Imagine a french door being replaced by a window. The previously non-existent section that will form the wall under the window needs to be constructed. The window will sit on the external leaf.

    Could I have (from outside in):

    - Render
    - External leaf blockwork
    - Insulation
    - Internal plaster

    ... maximising the insulation?

    Is a gap required between the blockwork and the insulation?

    The current surrounding blockwork is:

    - 100mm block
    - 50mm cavity full filled with mineral wool
    - 100mm celcon

    (The above is according to the architect drawings I managed to convince planning let me see... gawd knows what was actually done!)
    • CommentAuthortony
    • CommentTimeOct 5th 2018
     
    Good idea to maximise insulation, indeed you could simply render your insulation both sides!
    • CommentAuthorgravelld
    • CommentTimeOct 5th 2018
     
    Posted By: tonyGood idea to maximise insulation, indeed you could simply render your insulation both sides!
    The external skin is supporting the window though. That much Compacfoam would be pricey :shocked:

    Is there a structural reason the inner skin would still be required, and/or does the external skin have to be thicker?
  4.  
    Anything above the old french doors will be supported by the existing lintel. If the new window is supported by the outer skin then IMO there is no need for an inner skin and the outer skin only needs to be strong enough to support the window.
  5.  
    If a lot of the load of the window is taken on the vertical stiles, might not high-density EPS such as used in some below-DPC EWI, be sufficient? Much less dense and expensive than Compacfoam.
  6.  
    If a lot of the load of the window is taken on the vertical stiles, might not high-density EPS such as used in some below-DPC EWI, be sufficient? Much less dense and expensive than Compacfoam.
    • CommentAuthorgravelld
    • CommentTimeOct 5th 2018
     
    It's a good point - the windows are hung on the side reveals aren't they...?
    • CommentAuthorgravelld
    • CommentTimeOct 5th 2018
     
    One problem with just having insulation is the discontinuity of material for the external render which might cause cracking due to different reactions to thermal change?
    • CommentAuthorgravelld
    • CommentTimeOct 31st 2018
     
    Here's what I was thinking... am I going to be laughed at (by a builder or a BC officer) for suggesting this?
      rebuilt_cavity.jpg
    • CommentAuthortony
    • CommentTimeOct 31st 2018
     
    It will work, you will get a crack where the plasterboard joins the inner skin, I would set it back 10 or 15mm back from the face of the wall or line the whole wall with new plasterboard with no join under the window reveals
    • CommentAuthorgravelld
    • CommentTimeNov 1st 2018
     
    It's a big wall so I'd like to avoid lining the whole wall!

    It's currently wet plastered.

    Posted By: tonyI would set it back 10 or 15mm back from the face of the wall
    Do you mean and leave it inset, or then level flush with the existing wall (and if so, how?)
    • CommentAuthortony
    • CommentTimeNov 1st 2018
     
    It is easy to control a crack, and you will get one, down an internal corner so yes set back the bit under the window a tad.
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