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Housing - Renovation: EWI detailing

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  1.  
    I'm learning here - isn't the seal required to stop rain running down the outside of the render from tracking back through the gap between render and pipe, then making the masonry wet behind the render/insulation? (Or the straw in DJH's case)
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      CommentAuthordjh
    • CommentTimeJun 22nd 2019
     
    In my case, rain can saturate the render in any event, and so saturate the ends of the straw embedded in the render. But lime spreads water quickly so any excess liquid runs down and drains out the bottom and the water absorbed within the lime starts to evaporate as soon as the source goes away (i.e. it stops raining). So whilst cracks and suchlike are viewed as bad since they allow rainwater a direct path behind the render, I don't think that small ones are a real problem.

    It's all a balance. There's a famous case on a very exposed Brittany coast with lots of driven rain where the straw became saturated. I think that was rectified by adding external timber rainscreen cladding, after replacing some of the straw. The oft-quoted rule of thumb is that straw will cope with rain that strikes the side of a stack of bales, but not with rain that falls on top.

    In the case of EWI, there's no great problem with water in and on the back face of the render AIUI, as long as it isn't lifting the render away from the insulation, and as long as there's nothing that can track the water back through the insulation. Hence all pipes etc slope downwards as they exit. Of course the more water that is shed off the front of the render, the better.
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      CommentAuthorfostertom
    • CommentTimeJun 23rd 2019
     
    The render, its back face and the outer layer of the straw, EPS or wood fibre behind it will often be full of liquid water, even if no leaks, due to interstitial condensation, for much of the winter. Seems no problem as long as the render is water permeable and all can dry out as soon as conditions permit.
  4.  
    I don't think there should be liquid water in the wall... Interstitial condensation is a relatively small quantity, grammes per m2. For straw and woodfibre, the wicking effect would redistribute areas of raised RH through the thickness of the insulation, even to the evaporation point on the inside of the insulation. For EPS, the render is more permeable than the EPS, so water vapour that can diffuse through the EPS will easily diffuse out of the render rather than condensing, and the materials are not damaged by water.

    However, water leaks round pipes involve a lot more water, grammes per cm2, enough to saturate woodfibre/straw etc, and would deliver the water behind the EPS insulation and into the masonry or interior layers.
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      CommentAuthordjh
    • CommentTimeJun 23rd 2019
     
    Posted By: WillInAberdeenFor straw and woodfibre, the wicking effect would redistribute areas of raised RH through the thickness of the insulation, even to the evaporation point on the inside of the insulation.

    I've been looking for published evidence of this behaviour for quite a while now. Have you found some?

    However, water leaks round pipes involve a lot more water, grammes per cm2, enough to saturate woodfibre/straw etc, and would deliver the water behind the EPS insulation and into the masonry or interior layers.

    I'm not clear what scenario you're describing here; can you clarify? Liquid water runs downhill and if the pipes slope downwards, that water will be carried outwards, not inwards.
  6.  
    I haven't looked for original research on woodfibre or straw (we used neither in the last place) but there's lots of summary info online from the vendors and from others such as Lstiburek / Straube
    Eg

    https://www.buildingscience.com/documents/digests/bsd-138-moisture-and-materials

    The physics seem simple enough- woodfibre absorbs water vapour in a (non linear) relationship to RH, so takes up water in high RH areas (such as the outdoors side of a wall), desorbs it in low RH (inside), and redistributes it between them by capillary action.

    Haven't looked into straw but assume it's similar - this looks good:
    "significant proportion of drying can occur to the inside in strawbale walls" https://www.buildingscience.com/documents/digests/bsd-112-building-science-for-strawbale-buildings


    Re the pipes: Water (in our painful experience) will track helically around a drainpipe, so start on the top side of the 'downhill' end and end up at the bottom side of the 'uphill' end.
    •  
      CommentAuthordjh
    • CommentTimeJun 26th 2019
     
    Posted By: WillInAberdeenThe physics seem simple enough- woodfibre absorbs water vapour in a (non linear) relationship to RH, so takes up water in high RH areas (such as the outdoors side of a wall), desorbs it in low RH (inside), and redistributes it between them by capillary action.

    That's the kind of mechanism I'm looking for but I haven't found any evidence yet.

    Haven't looked into straw but assume it's similar - this looks good:
    "significant proportion of drying can occur to the inside in strawbale walls" https://www.buildingscience.com/documents/digests/bsd-112-building-science-for-strawbale-buildings

    The particular quote you chose is about vapour diffusion-driven drying. In regards to capillary action it says: "... reduced capillary suction, e.g., strawbales do not “wick” water very effectively. The very limited capillary suction of straw means that a separate capillary break in the form of building paper is not strictly necessary (water will definitely not wick from the exterior stucco to the interior!)." i.e. a flat contradiction.

    Re the pipes: Water (in our painful experience) will track helically around a drainpipe, so start on the top side of the 'downhill' end and end up at the bottom side of the 'uphill' end.

    Ah, OK, thanks for the explanation. I presume the water needs to find something to absorb it at the inner end, otherwise it will just run back out down the bottom of the pipe?
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      CommentAuthorfostertom
    • CommentTimeJun 26th 2019
     
    V interesting
    •  
      CommentAuthordjh
    • CommentTimeJun 26th 2019
     
    Posted By: WillInAberdeenRe the pipes: Water (in our painful experience) will track helically around a drainpipe, so start on the top side of the 'downhill' end and end up at the bottom side of the 'uphill' end.

    Thinking about it a little more. In our case the ventilation ducts run through blown-in cellulose insulation, so I expect any water that does get behind the terminal and through the render will be promptly absorbed by the cellulose rather than managing to track back further. So I'm still fairly relaxed about our construction, but I accept the possibility should be thought about.
    • CommentAuthorwookey
    • CommentTimeMay 3rd 2020
     
    <blockquote>Water (in our painful experience) will track helically around a drainpipe, so start on the top side of the 'downhill' end and end up at the bottom side of the 'uphill' end. </blockquote>

    Doesn't that need some reason for the water to track towards the interior (such as an airgap with air moving inwards or a pressure difference), otherwise it should just fall down and end up at/near the bottom of the bottom end. Or is drier material to the inside sufficient - but then isn't that just capiliary action, rather than tracking round the pipe?

    I see the theoretical issue (and you say you had it for real), but I'm failing to see the mechanism.

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