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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorukproptop
    • CommentTimeJan 18th 2018
     
    I am going round in circles trying to get a solution to this problem. Any input / ideas would be very gratefully received.
    The project is a big barn conversion. We have a dwarf wall around the perimiter with 24 piers that stick in further. On top of the dwarf wall is stud work that will be insulated with quilt. The problem is how to insulate the dwarf wall. Once the screed & floor insulation is down it will only be about 400mm high but I still need to avoid any condensation / damp problems.
    There is no DPM under the slab so it will go under the floor insulation. We then need to bring this up the walls so it can be taped to an airtight membrane.
    To get any half decent join taped up we need a nice clean DPM edge to tape the intelli pro to. The problem is because the piers (all 24 of them) each with 2 outside and 2 inside corners it will be horrible with all the taping and cutting and inevitably 'sub-optimal'. Additionally still just inside the perimeter wall are 24 new posts for the roof that the DPM will also need to go around (unavoidable). This means we can't batton out the dwarf wall much even if we didn't mind losing the floor space.
    My idea was to pad out the recess between the dwarf walls with PIR insulation and then have a nice clean edge wall to run the DPM up. I.e. the main body of insulation would be outside the DPM. We could then run another 25mm of PIR on the inside of the DPM overlapping the higher stud to make sure no obvious cold bridge across the sole plate.
    I am told that it is a 'bad idea' to have the 100mm DPM outside the DPM. It would fully fill the void and there would still be a clear run of 25mm on the inside of the DPM.
    The blockwork has been damp. But we are on sand, gutters have only just gone on, we have Black-Jack DPMed the whole dwarf wall on the inside, will seal it on the outside and will dig a trench and shingle all around the outside wall so it stays as dry as possible. So is it really a risk of damp coming in? I don't think so.
    So then it is only potential condensation that is an issue. We are Vapour controlled on the inside with the air-tight membrane and there shouldn't be a cold bridge, although the piers will only have 25mm of PIR insulation.
    Do I have to worry? Given it is only 400mm high?
    If i can't do as I hope the only alternative is a lot of fiddly DPM cutting and taping which will have its own problems as well as a hard job getting an edge to bring the airtight membrane taped onto. I can't see which is worse.
    Any ideas or advice very gratefully received! Thank you
    •  
      CommentAuthordjh
    • CommentTimeJan 18th 2018
     
    A plan and a sectional drawing (sketches) would probably be useful?

    If you've painted a DPM on the inside of the dwarf wall, and then plan to run another DPM up the inside of your insulation, doesn't that leave the insulation sandwiched by two DPMs? It's generally best to leave some way for (1) water vapour and (2) liquid water to escape from every space, since it has a tendency to get into every space eventually somehow.
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