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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorgoodevans
    • CommentTimeSep 29th 2018
     
    At our gable ends we will have some bits of wall that the plasterers will not get to when they wet plaster. In particular the bit of wall where the joists are located.

    I have a single skin block wall - I could put in the famous Tony Tray - but that would leave a 300mm strip of membrane for the EWI to span over on the outside.

    Instead I was thinking of parge coating the inside of the wall - before the ceiling board goes up. I could use somthing like Gyproc Soundcoat Plus, or for more a more flexible coat something like Parex Maite. Both applied at about 6mm thick. I was hoping that embedding a mesh would be overkill

    Any thoughts?
    •  
      CommentAuthordjh
    • CommentTimeSep 29th 2018
     
    I'm not sure I entirely understand. What is your worry? Is the internal plaster your airtight barrier? Why is the EWI spanning 300 mm membrane a problem? If it really is a problem, why can't you anchor the EWI to the wall through the membrane?

    FWIW, we chose to run a membrane internally, bolt a ledger beam to the wall through that membrane and hang the joists from the ledger beam.

    If you parge coat the wall (around the joists, presumably?) you will have the typically retrofit task of sealing the joists to the parge coat, which I understand can be a bit tedious.
    • CommentAuthorgoodevans
    • CommentTimeSep 30th 2018
     
    We have attic joists running parallel to the gable wall - so the only interaction between the wall and the joists is the wall bracing.

    I plan to just use Parex Maite as the EWI adhesive without anchors - but I could anchor locally where membrane is located.

    The issue with a membrane barrier is that It can also act as a air path conduit - that's fine provided all holes, gaps and penetrations are sealed - which they should be. On the other hand a parge coat/wet plaster has the benefit that it blocks most of the air infiltration with ease but may struggle with the last tricky bits (e.g. joist penetrations) - It also has a problem if the wall moves/cracks.

    In effect I will have a reasonable parge coat on outside surface of the wall in the form of the EWI adhesive - so leakage on the gable walls will probably be small - and would be reduced further with an internal parge coat.

    For the eaves walls I have a different detail which uses membrane which will need attention during implementation to ensure a good seal.
    •  
      CommentAuthordjh
    • CommentTimeSep 30th 2018 edited
     
    Posted By: goodevansWe have attic joists running parallel to the gable wall - so the only interaction between the wall and the joists is the wall bracing.

    may struggle with the last tricky bits (e.g. joist penetrations)

    I'm confused. I thought you just said you don't have joist penetrations? How would you use a Tony tray if the joists are parallel?

    The issue with a membrane barrier is that It can also act as a air path conduit - that's fine provided all holes, gaps and penetrations are sealed - which they should be. On the other hand a parge coat/wet plaster has the benefit that it blocks most of the air infiltration with ease but may struggle with the last tricky bits (e.g. joist penetrations) - It also has a problem if the wall moves/cracks.

    I don't understand what you mean by an air path conduit. The purpose of the membrane is to block air flow, as is that of a parge. Movement of an entire wall is not too likely or troubling; it is movement at the high stress concentrations that is the concern - edges, corners and points. Either using membrane/tape at these places or reinforcing the plaster, or both, are the usual techniques to allay such concerns.

    In effect I will have a reasonable parge coat on outside surface of the wall in the form of the EWI adhesive - so leakage on the gable walls will probably be small - and would be reduced further with an internal parge coat.

    Conventional wisdom says that you should concentrate on one surface nominated as your airtightness barrier. Attention to detail is key regardless.
    • CommentAuthorgoodevans
    • CommentTimeSep 30th 2018
     
    My guess is that I've been overthinking this - I just need to join the plasterwork on the ground floor with the plasterwork on the first floor. The easiest way to do this is to slap on a parge coat before the ceiling boards go up.

    Because that parge coat is going to be covered up - I'll spend the extra and use the Parex Matie as it should be more flexible.

    I think If I could rewind a day I would leave out the middle paragraph of the original post.
    • CommentAuthorgravelld
    • CommentTimeSep 30th 2018
     
    Could blowerproof be a good solution? Very flexible. Apparently increasingly popular in retrofits.
    • CommentAuthortony
    • CommentTimeSep 30th 2018
     
    I don’t think parge between the joists will work, the joists are sure to shrink and open up gaps and cracks that could let in outside air and wind in an area that is difficult to access to rectify.
    •  
      CommentAuthordjh
    • CommentTimeSep 30th 2018
     
    I think we just established that the joists run parallel to the wall under consideration. That is, there are no joist penetrations to consider. If there were then the established practice is to seal the joint with tape.
    • CommentAuthorgravelld
    • CommentTimeOct 2nd 2018
     
    Doesn't it still get tricky, though, at the corners where multiple timbers are connected, you have the wall below meeting it...
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