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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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    • CommentAuthorgravelld
    • CommentTimeJan 7th 2018
     
    If you use plaster (or render) as an air barrier, and you want to combine both new and existing coverings to create one air tight layer, how should the junctions between the two be treated? Because they will inevitably crack.

    My example is existing render - I want to parge coat the plinth and foundations and un-rendered top of eaves for example, and meet the existing render on the visible portion of the outside wall.
  1.  
    Use a plaster-on tape - Pavatex and Pro Clima make them, as I am sure do others. Overlap the plaster/render coats, but with the tape behind to reinforce the joint.I think Tony has just used expamet for some junctions.
    • CommentAuthortony
    • CommentTimeJan 7th 2018
     
    Nope, I used expamet over my ceiling or tony tray air barriers as when plastered in these produce a hermetic seal

    If I wanted to join render to plinth render presumably over a dpc, then strip of air barrier 100mm wide and 150 elm over it with bell cast drip bead over that at dpc level.
    • CommentAuthorgravelld
    • CommentTimeJan 7th 2018
     
    Posted By: Nick ParsonsUse a plaster-on tape - Pavatex and Pro Clima make them, as I am sure do others. Overlap the plaster/render coats, but with the tape behind to reinforce the joint.I think Tony has just used expamet for some junctions.
    I don't understand - you can tape behind the new coat, but what about the existing coat? Does it work like an S shape, one half on top of the old plaster, the other half under the new plaster?
  2.  
    ''I don't understand - you can tape behind the new coat, but what about the existing coat? Does it work like an S shape, one half on top of the old plaster, the other half under the new plaster?''

    I would do a first coat of new plaster, sitting say 10mm below face of old plaster, prime that and the exg plaster with a specialist air-tightness primer (available from suppliers previously mentioned) and tape over the 2 elements, then build up new plaster coat to exceed depth of exg plaster and 'feather' new plaster on to exg plaster. Is that clearer? Sorry, I can't scan docs at present, or I'd do a sketch.
    • CommentAuthorgravelld
    • CommentTimeJan 8th 2018
     
    Yes, that's perfectly clear thanks. Although why not tape from the beginning, assuming the original sub surface can be suitably primed? I'm just thinking of the case of parge coating which is normally just one coat (AIUI).
  3.  
    ''Although why not tape from the beginning, assuming the original sub surface can be suitably primed?''

    You are right to Q that - It depends on the substrate. If (say, up near the eaves) it's brickwork with poor pointing, or 'rumply' stone-work, the chance of a good 'stick' with no gaps is very small, hence the 'pre-parge' coat - just a layer to stick to.
    • CommentAuthorbhommels
    • CommentTimeJan 8th 2018
     
    Hi, for the new airtight extension meeting the old, drafty house, the seams are sealed by putting Siga Fentrim tape across the joints, making sure to have a stress relief fold in the tape to avoid it ripping upon the slightest movement. Where needed because of poor surfaces I slapped on some Thistle undercoat plaster to provide a good surface/key for the tape to stick to. BTW for wall-floor joints, some leftover tile adhesive worked very well to anchor down the tape to the screed floor.
    Tapes were generally buried in the render layer, just a plaster skim will probably not be strong enough to keep the tape in place.
    • CommentAuthorgravelld
    • CommentTimeJan 8th 2018
     
    Thanks, less familiar with the Siga products.
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