MortarThePoint

Insulating Around Cavity Trays

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Cavity trays made using DPC roll are often shown simplistically crossing the cavity at an angle from their top on the inner leaf to bottom on the outer leaf. In reality, even if you try to run the DPC material down the inner leaf before crossing the cavity, you will still create a void behind the cavity tray that will end up uninsulated:

 

image.thumb.png.49fa335de70a4d4b61c21686dbfe90cf.png

 

Is there a way to insulate this void or is it a matter of trying to keep it as small as possible? Preformed corners and stop ends all seem to adopt the shape on the left and so will create quite a significant cold bridge.

 

I'm using blown bead insulation. There is no route for the beads to get between the gas membrane and cavity tray. Also I wouldn't want the pressure of the bead blowing to dislodge the cavity tray.

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I’m not a fan of trays around the bottom of a building 

Do you really need a tray at DPC level ? 
Or was it included by the Architect like ours 

I decided not to have a perimeter tray for the reason that you have highlighted 

 

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Just now, nod said:

I’m not a fan of trays around the bottom of a building 

Do you really need a tray at DPC level ? 
Or was it included by the Architect like ours 

I decided not to have a perimeter tray for the reason that you have highlighted 

 

We have a gas barrier membrane crossing the cavity and as I understand it the rule is that anything bridging the cavity has to have a cavity tray above it. It's a pain, but I think we have to have it.

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25 minutes ago, MortarThePoint said:

 

We have a gas barrier membrane crossing the cavity and as I understand it the rule is that anything bridging the cavity has to have a cavity tray above it. It's a pain, but I think we have to have it.

Ah I see the membrane must be above dpc then 😂

No gas membrane on ours 

Edited by nod

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Get some eps 100 sheets and cut it to shape, easy with a hand saw, have them pre cut and fit them as your brickies go along. 

 

You could get them pre cut at the eps manufacturers if you where happy to do a bit of a mock up. 

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I don’t have a gas membrane but Foam  insulation below and rockwall above the angled tray, we simply cut angled pieces with a hand saw for below, boring but quite easy, does not need to be millimetre perfect.

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There is a company that makes some insulation for use with cavity trays:

    https://www.jackon-insulation.co.uk/products/detail/jackodur-cavity-tray-insulation/

    image.png.20303eb0fd08904309d6bedf9e6ad26f.png

 

I wouldn't be able to get these in time. I wondered about using polystyrene coving and pinning the cavity tray to the face of the inner leaf as then there would only be the curve as it makes a single transition from vertical to horizontal. Then I saw the coving is hollow so wouldn't help much.

 

I'll have to make sure the cavity tray is installed loose enough for the blown beads to push it down onto the face of the inner leaf without applying a tension to the cavity tray and risking dislodging it. With a bit of care, it should be possible to have a radius of curvature of about 25mm or less at both the top and the bottom of the vertical portion of the cavity tray (as shown on the right side of my sketch) and so the missing amount of insulation will be small (about 10% over a height of 25mm). If the DPC material can actually be convinced to fold then it will be a lot less.

 

450mm is a standard width for DPC but that is a bit tight to make this out of. 102mm outer leaf, 100mm cavity, and 225mm up inner leaf leaves only 23mm stuck in to the inner leaf blockwork bed. Is that enough? If tight and crossing at an angle making maximum cold bridge, it would have about 100mm available for the inner leaf bed. I think I'll get my scissors out and trim down a load of 600mm wide DPC to 500mm.

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On 01/08/2020 at 11:11, nod said:

Ah I see the membrane must be above dpc then 

 

I might be missing something here. The gas membrane is at DPC level of the outer leaf and I am forming the DPC using the cavity tray that is on the surface of the membrane.

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I don’t see the problem, drive to the builders merchants and collect eps sheets, cut to size on site and hand to the brickies. 

 

Done, the stuff jackodor sell is xps you can get that at some insulation stores but maybe an order job, eps is available in most builders merchants and even in wickes. 

 

If it wont fit in the car then use there cavity boards that are 1200x 600. 

 

Easy peasy lemon squeezy. 

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