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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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  1.  
    More researching turned up a psi value in SAP2012 app K for a bridge from an EWI-ed room up through the gable masonry and into a cold loft, the default value is 0.48W/mK.

    In the example above, this would give a heat loss of 7m X 10 Deg X 0.48W/mK = 34W, which is worse than my guesstimate. So the heat loss up through the cold bridge is rather worse than the heat loss out through the EWI. (!)

    The psi value is not specifically for a cwi wall with ewi, bit of a niche application..

    There is also an 'accredited detail' on the English govt planning portal, showing rafter insulation extending over the top of the gable to join the EWI, this reduces the cold bridge to 0.04W/mK. Interested whether anyone here has constructed this 'accredited detail'?
      Screenshot_20190508-000000.png
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      CommentAuthordjh
    • CommentTimeMay 8th 2019
     
    I like where it says 'the use of under rafter insulation is considered best practice ...'

    Pity there isn't [I assume] a similar statement for under joist insulation in crawl spaces !
    • CommentAuthorgravelld
    • CommentTimeMay 8th 2019 edited
     
    Well done Will it sounds like an ambitious project.

    I'm not sure I quite understand your comparison djh (the equivalent would be insulation over the joists wouldn't it?) but I think it means "best practice in this situation", i.e. if insulating above and between the rafters isn't an option.
    •  
      CommentAuthordjh
    • CommentTimeMay 8th 2019
     
    Posted By: gravelldI'm not sure I quite understand your comparison djh (the equivalent would be insulation over the joists wouldn't it?) but I think it means "best practice in this situation", i.e. if insulating above and between the rafters isn't an option.

    The point is that in another thread we are discussing whether BR mandates NOT insulating either under or over said joists.
    •  
      CommentAuthorfostertom
    • CommentTimeMay 8th 2019 edited
     
    Posted By: WillInAberdeenThe place we are considering actually has a room in the roof, so we would insulate the roof between/below the rafters, joining this onto the EWi at the eaves, the question is how to join the roof insulation to the EWI at the top of the gable?
    Why not run your EWI up the outside face of the walls then unbroken on OSB over the top face of the rafters? The join-up at both eaves and gable verge becomes perfect, easy, foolproof tea-cosy.

    Then how to do the rafter overhang at both eave and verge? Stub purlin-ends fixed on top of the OSB and stub rafter-ends laid on top of the purlins, all with the roof EWI (RWI?) fitted around.
      P1010283med.JPG
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      CommentAuthorfostertom
    • CommentTimeMay 8th 2019
     
    finished
      P1000259med.JPG
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      CommentAuthordjh
    • CommentTimeMay 8th 2019
     
    Posted By: fostertomWhy not run your EWI up the outside face of the walls then unbroken on OSB over the top face of the rafters?

    Because he said he's not planning to take the roof off, because he doesn't expect to get permission to raise the height.
    • CommentAuthorwookey
    • CommentTimeMay 9th 2019 edited
     
    Hmm. Good point about the gable end thermal bridge to the loft. Not an issue with IWI, but it is for EWI. Cutting-in a row of insulating blocks does seem like the right thing to do (same with a chimney). I should do the sums properly for here.

    Nick - I agree the plastic dry verge systems look pretty horrid.

    Do building regs mean that one really should be switching to some kid of mechanical edge fixing? Or if this one of those 'so long as it's no worse than it was before it's OK' situations?
    •  
      CommentAuthorfostertom
    • CommentTimeMay 9th 2019
     
    Posted By: djhbecause he doesn't expect to get permission to raise the height
    Did he say that? With 150 EPS on 11OSB over the rafters, plus an extra layer of downslope battens, 186 at say 45o pitch raises the ridge by 263 and the eaves not at all, because they're extended outward therefore downward as well.
  2.  
    Nice photos Tom thanks! It's early days yet on potential next project house, going for a second viewing. The house is already rendered so ewi suggests itself. It has a rather attractive patterned slate roof, presumably early 20th C, worth preserving. There's plenty of unknowns already so we don't want to be 100% reliant on getting permissions, nice to know that there would be a way it could be done without changing appearance, if necessary.

    I haven't yet found examples of where existing house has been EWI'd and the gable EWi joined to under-rafters insulation - does nobody do this because it is difficult/overkill/ some other reason?
    •  
      CommentAuthordjh
    • CommentTimeMay 9th 2019
     
    Posted By: fostertomDid he say that?


    Posted By: WillInAberdeenUnlikely to be permitted to raise the roof line to insulate on top of the existing roof structure.


    Posted By: WillInAberdeenAgreed it would be best to tea-cosy over the whole building but at the mo it doesn't look like we will be stripping the roof off.


    Have you lost the ability to reread, Tom?
  3.  
    Posted By: WillInAberdeenI haven't yet found examples of where existing house has been EWI'd and the gable EWi joined to under-rafters insulation - does nobody do this because it is difficult/overkill/ some other reason?

    some other reason(s)
    1 They don't know/appreciate the issues of the cold bridge
    2 Contractors doing the EWI don't want to get involved with fiddly non-core insulation work. They just want to stick up the EWI quickly (usually in my experience with less that the recommended thickness of adhesive and render) and move on to the next job.
    3 cost vs. perceived benefit
    •  
      CommentAuthorfostertom
    • CommentTimeMay 9th 2019 edited
     
    Posted By: WillInAberdeenI haven't yet found examples of where existing house has been EWI'd and the gable EWi joined to under-rafters insulation - does nobody do this because it is difficult/overkill/ some other reason?
    We did it - gable end EWI joined to quilt insulation at ceiling level, where recently done slate roofing had to be left undisturbed.
    We did ruff'n'ready EWI on the inner face of the gables and stacks.
    Also, working from inside, enlarged extg space within extg rafter + purlin depth, removing upstanding stones from the top edge face of the gables masonry and capped it with EPS. Well, the substantial chimey stacks (long heat-loss path) punctured that top-edge insulation but at least the triangles of gable masonry closest to the eaves (short heat-loss path) were capped.
      2011-10-31 045.jpg
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      CommentAuthorfostertom
    • CommentTimeMay 9th 2019 edited
     
    BTW, see the small-bore UFH pipes running out from manifolds in the loft space, to zigzag down the outside of the extg walls and back, all thermally bedded onto the wall and covered with the EWI.

    They are fed by mixed tepid water, which stores masses of heat into the whole external wall envelope of 500mm rubble stone, which radiates into the interior at rock-solid temp that only changes on seasonal timescale. Water flow temp is only 2-3C above 18C internal temp, and internal wall face is about 1C above room temp.

    We'd planned to feed the pipes from wet solar panels optimised for mid winter solar angles, which can collect with enormous efficiency up to 60%, at such v low flow temp. The intermittency of mid winter fine sunny days was OK because the walls' storage capacity could bridge that.

    In the end, it's still fed from a biomass/heat store, but is much appreciated because of the v comfortable ultra-high-radiant-%age internal environment, which allows comfort air temp to run as low as a v un-sweaty 18C.
      2011-10-31 043.jpg
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      CommentAuthorfostertom
    • CommentTimeMay 9th 2019 edited
     
    The zigzagging pipes, burried in parge before EWI fixing. This was before feed-in tariffs, before PV became remotely affordable.

    Nowadays, wd consider doing it with PVs feeding elect resistance tapes, gets over massive complication of pipes, manifolds etc as heat transport medium - but against that PVs can't so far even begin to match the collection efficiency of wet solar, so wd need multi-times more collector area to work with meagre mid winter input.
      IMG00265-20101210-1441smaller.jpg
  4.  
    Posted by FT: " gable end EWI joined to quilt insulation at ceiling level, where recently done slate roofing had to be left undisturbed... Also, working from inside, enlarged extg space within extg rafter + purlin depth, removing upstanding stones from the top edge face of the gables masonry and capped it with EPS. "

    Tom, that's a work of art!

    Thanks v much indeed for that! it would solve a number of problems. We could insulate over the top of the masonry wall whilst insulating the rafters, and the EWI people could come later and join on to it, without doing anything out of their norm as PiH described.

    The wall heating is interesting too, liked the closer pipe spacing downstairs, nice touch. If you embedded another loop in the outer render layer of the EWI and painted it dark, you could use it as a midwinter-angle solar collector for a heat pump to heat the loop in the parge coat?
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