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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorlineweight
    • CommentTimeApr 29th 2020
     
    If you look at most standard details for cavity trays, the type that full span the cavity and are fixed into the inner leaf, they either ignore the insulation completely, or they are drawn as a line passing through it, as if the insulation boards below and above it are cut to its exact slope leaving no gaps. Of course that's very unlikely to be what actually happens on site. It seems to me that with any cavity tray that is fixed into the inner leaf, there's inevitably going to be an interruption to the insulation layer and a cold bridge.

    There are the 'self supporting' type that don't need to fix into the inner leaf, and where you've got a partial fill cavity, that works fine (see upper diagram in the image below).

    With full fill, to use one of these, you can notch out the insulation (although this doesn't seem to be that much of a widely recommended detail) and at least that means it's not fully interrupted, but it's still reduced locally, and it also seems to invite shortcuts on site where it actually ends up discontinuous.

    Does anyone know of any better solutions for full fill cavity solutions? Or is this just an inherent weakness in full fill cavity construction?

    (image is from a Timloc technical doc)
      Screen Shot 2020-04-29 at 10.54.24.jpg
    • CommentAuthortony
    • CommentTimeApr 29th 2020
     
    You might look at this http://tonyshouse.readinguk.org/air-sealing-windows-and-doors/

    I used two seperate lintels. L10 outside, solid concrete or rsj inside 300,, cavity insulation with dpm supported on plastic sheet , held against the inner skin by the insulation and tucked in to the back of the
    L10 , no weep holes, no problems, 10 years on

    You might catch a glimpse on the videos
    • CommentAuthorlineweight
    • CommentTimeApr 29th 2020 edited
     
    I guess I'm talking more about intermediate situations - ie. not situations where the cavity tray is immediately above a lintel, because in that situation there's by necessity a discontinuity in insulation and also something to support a sheet material type cavity tray.

    For example where there's a roof flashed into a vertical wall. Or certain parapet conditions.

    Also something that is reasonably standard ie most builders happy with it, and satisfy things like NHBC standards which I think would demand eg. weepholes.
    • CommentAuthorMike1
    • CommentTimeApr 29th 2020
     
    You're right, that is the typical illustration - but, provided the tray has sealed stop ends so water can't escape into the cavity, an all-vertical & horizontal tray works equally well.

    There's an illustration of that in Building Regs Part C - see diagram 9.
    • CommentAuthortony
    • CommentTimeApr 29th 2020
     
    I have always built cavity trays into full fill insulation with no gaps, for stepped flashings with cavity trays each tray tips any drips or water down hill into the tray below it. If for example mineral wool batts get damp they dry out fairly soon and can’t track wet across the cavity.

    More difficult to detail with sheet insulation.

    Never had any problems even without using weep holes, water seeps out
    • CommentAuthorlineweight
    • CommentTimeApr 30th 2020
     
    Posted By: Mike1You're right, that is the typical illustration - but, provided the tray has sealed stop ends so water can't escape into the cavity, an all-vertical & horizontal tray works equally well.

    There's an illustration of that in Building Regs Part C - see diagram 9.


    By all-vertical and horizontal tray, do you mean it crosses the insulation layer horizontally rather than on a slope, therefore easier to fit rigid boards above and below it?

    (The building regs diagram doesn't give any suggestion as to how insulation is incorporated)
    • CommentAuthorlineweight
    • CommentTimeApr 30th 2020
     
    Posted By: tony
    More difficult to detail with sheet insulation.




    Yes, maybe I should have said in original post, I'm thinking mainly of scenarios with rigid insulation.
    • CommentAuthorMike1
    • CommentTimeApr 30th 2020 edited
     
    Posted By: lineweightBy all-vertical and horizontal tray, do you mean it crosses the insulation layer horizontally rather than on a slope, therefore easier to fit rigid boards above and below it?

    Yes :)
    • CommentAuthorlineweight
    • CommentTimeApr 30th 2020 edited
     
    Posted By: Mike1
    Posted By: lineweightBy all-vertical and horizontal tray, do you mean it crosses the insulation layer horizontally rather than on a slope, therefore easier to fit rigid boards above and below it?

    Yes :)


    Does such a thing exist as a product though? Because if it does I can't find it!

    There is eg. this from Glidevale, but it is not intended to cross the insulation layer.
      Screen Shot 2020-04-30 at 17.53.17.jpg
    • CommentAuthorMike1
    • CommentTimeApr 30th 2020 edited
     
    There's no rule that says you must a pre-formed product. You can use a wide DPM (calculate the width you need) such as https://www.builderdepot.co.uk/450mm-iko-hyload-original-dpc-damp-proof-course-20m and cut & fold it to shape, including the stop ends (which only need to be big enough to stop the water spilling off).

    Only a very basic level of origami is required :)

    Any lead flashing would need to be separate, formed & fixed using traditional techniques.
  1.  
    Posted By: Mike1There's no rule that says you must a pre-formed product. You can use a wide DPM (calculate the width you need) such ashttps://www.builderdepot.co.uk/450mm-iko-hyload-original-dpc-damp-proof-course-20m" rel="nofollow" >https://www.builderdepot.co.uk/450mm-iko-hyload-original-dpc-damp-proof-course-20mand cut & fold it to shape, including the stop ends (which only need to be big enough to stop the water spilling off).

    Only a very basic level of origami is required :)

    Any lead flashing would need to be separate, formed & fixed using traditional techniques.


    Thanks. I expect that works fine and it's an approach I would likely take if building my own project... however in this case I'm looking at it from a specifier's point of view, which means I am always happier if I can find a specific recommendation amongst a supplier's literature that shows the product is intended to be used in the way that I am proposing. Firstly, so that I can show that I've done all that I can to ensure the detail is proven and will work, and secondly in case it is queried on site - so that I can point to some specific instructions rather than an "I reckon this'll be fine" approach, or in case the argument is made to me that this is not a recognised way of doing things, and is therefore impractical/too slow/expensive etc.

    So I looked at Iko literature, and indeed it states that the stuff can be folded to your chosen profile, but it says that for corners and and so on, that you should use their prefabricated elements, which look like this... and I can't help but notice that they all seem to be based on an assumption that the DPM takes a sloped path from inner to outer leaf.
      Screen Shot 2020-05-01 at 10.33.45.jpg
  2.  
    And there's a bit in NHBC giudance that says it's fine to form a stop end by folding a sheet material... but again the diagrams show that sloped profile.
      Screen Shot 2020-05-01 at 10.36.23.jpg
    • CommentAuthorMike1
    • CommentTimeMay 1st 2020
     
    Cavity Trays Ltd will produce bespoke units - I guess others will too.
    • CommentAuthorwookey
    • CommentTimeMay 2nd 2020
     
    Is it really too hard to chamfer the top/bottom of the insulation so that it butts up against the sheet material forming the cavity tray properly? That's the detail GBS put in for their DPCs: https://www.greenbuildingstore.co.uk/technical-resource/denby-dale-passivhaus-uk-first-cavity-wall-passive-house/#tab_2 (gah, stupid modern interwebs that don't let you link a specific image - 2nd pic under 'during construction' with a section of cavity wall).
    •  
      CommentAuthordjh
    • CommentTimeMay 2nd 2020
     
    wookey wrote: "Is it really too hard to chamfer the top/bottom of the insulation so that it butts up against the sheet material forming the cavity tray properly? That's the detail GBS put in for their DPCs:https://www.greenbuildingstore.co.uk/technical-resource/denby-dale-passivhaus-uk-first-cavity-wall-passive-house/#tab_2 (gah, stupid modern interwebs that don't let you link a specific image - 2nd pic under 'during construction' with a section of cavity wall)."

    You mean https://www.greenbuildingstore.co.uk/wp-content/uploads/cavity-wall-150x150.jpg ?

    (stupid forum that doesn't allow both quotes and links! - but I don't think there's a problem with the interweb :)
    • CommentAuthorlineweight
    • CommentTimeMay 3rd 2020 edited
     
    Posted By: wookeyIs it really too hard to chamfer the top/bottom of the insulation so that it butts up against the sheet material forming the cavity tray properly?


    I don't know - I've never tried to do it physically myself - but I don't think it's what commonly happens on building sites.

    To be clear - this particular question is not really about how one might do it in a self-build with full control over everything; I am interested in whether there are reliable "standard" solutions. Something that will have a good chance of translating from what's drawn on a drawing, to something that actually works as built on site.

    I think that what usually happens in reality is that there's simply a gap and a cold bridge at most cavity trays.

    (And this probably explains why the insulation is simply ignored when you look at a lot of cavity tray manufacturer literature)

    One particular situation is where you have a stepped cavity tray.

    I've come across this video which identifies the problem I'm on about:

    https://youtu.be/5fZlQPrQXZI
    • CommentAuthorMike1
    • CommentTimeMay 3rd 2020 edited
     
    Posted By: wookeyIs it really too hard to chamfer the top/bottom of the insulation so that it butts up against the sheet material forming the cavity tray properly?

    It's certainly possible, and is done in practice too, but it depends on how much you trust those on site to do it correctly.

    Posted By: lineweightI think that what usually happens in reality is that there's simply a gap and a cold bridge at most cavity trays.

    That may happen sometimes. I once worked for a company that (among other things) fixed defects under the NHBC scheme, when the original builders had failed to rectify them; after doing that, not much surprises me.
  3.  
    Posted By: Mike1
    Posted By: wookeyIs it really too hard to chamfer the top/bottom of the insulation so that it butts up against the sheet material forming the cavity tray properly?

    It's certainly possible, and is done in practice too, but it depends on how much you trust those on site to do it correctly.


    Have you seen it done in practice for a stepped flashing/cavity tray?
    • CommentAuthorMike1
    • CommentTimeMay 4th 2020
     
    No, not seen it on full cavity insulation. The last one I was involved with used partial cavity fill - 25mm insulation continuous against the inner leaf, with preformed trays spanning the 50mm unfilled portion of the cavity (so spanning far enough to catch drips from the wall ties).
    • CommentAuthorwookey
    • CommentTimeMay 12th 2020
     
    Thank you Dave, yes. Although this image is more useful size: https://www.greenbuildingstore.co.uk/wp-content/uploads/cavity-wall.jpg
    • CommentAuthorlineweight
    • CommentTimeMay 12th 2020
     
    Posted By: Mike1No, not seen it on full cavity insulation. The last one I was involved with used partial cavity fill - 25mm insulation continuous against the inner leaf, with preformed trays spanning the 50mm unfilled portion of the cavity (so spanning far enough to catch drips from the wall ties).


    That seems like the most reliable approach to me, in principle.
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