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Housing - Renovation: XPS as EWI in French drain

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      CommentAuthorfostertom
    • CommentTimeSep 19th 2012
     
    Lots of info here - Search for EWI - button at top of this page
    • CommentAuthornikhoward
    • CommentTimeSep 26th 2012
     
    Sorry if this is a bit thick, but would the EPS as EWI above and below ground not be bridging the DPC. I thourhht the norm was having a base rail at DPC level to separate and create drip detail?
    •  
      CommentAuthorfostertom
    • CommentTimeSep 26th 2012
     
    So they say, but can't say why. How would EPS cause bridging?
    • CommentAuthorHollyBush
    • CommentTimeSep 27th 2012
     
    Posted By: fostertomSo they say, but can't say why. How would EPS cause bridging?


    Capillary action maybe?
    •  
      CommentAuthorfostertom
    • CommentTimeSep 27th 2012
     
    If you dunk a slab of EPS half into water and leave it, does it soak upwards? Microscopically, AFAIK. Anyway, it drains/dries out again freely - which the generally recommended XPS doesn't.
    • CommentAuthorHollyBush
    • CommentTimeSep 27th 2012 edited
     
    Dunno about EPS, but generally speaking if you put two surfaces close to each other and in water they will draw up the water, glass is good at this.

    I think any substance bridging the DPC has the potential to bring damp up through capillary action - the absorbency of the substance, the joints, and adhesive will all affect the speed/height.

    I too would like to know the risk though, as I am about to build a single skin extension and use EPS above the DPC and probably XPS below, but might still use EPS and was wondering about the DPC issue. Given that you can use engineering bricks instead of the black stuff, maybe I shouldn't worry?
    •  
      CommentAuthorfostertom
    • CommentTimeSep 27th 2012
     
    Capillary gaps capable of drawing up water need to be narrow and continuous - the gap between a slab of EPS and a masonry surface wd have such varying gap, due to the texture of both - and so many capillary breaks - as to be ineffective I'd have thought. That's different from water being drawn up (or not) thro the more consistent, and narrow gap/passages within a slab of material.
  8.  
    Posted By: nikhowardSorry if this is a bit thick, but would the EPS as EWI above and below ground not be bridging the DPC. I thourhht the norm was having a base rail at DPC level to separate and create drip detail?


    Some systems use a plastic bead to act as a DPC that breaks both the insulation and the render at the appropriate level. Not sure if all systems are supposed to have this or not.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 29th 2012
     
    Tom
    You could use that capillary action to run a nano turbine, there are Coulomb forces and Van de Waals', how useful is that.

    Height = (2 x surface tension x cos theta) / (density x gravity x radius)
    • CommentAuthornikhoward
    • CommentTimeSep 29th 2012
     
    Render will wick up moisture from the damp blocks below DPC to the dry ones above and cause damp

    In Cornwall it is quite normal that cement render bridges a DPC and Cornwall is quite damp. Also I have been building for quite a few years but been in Cornwall for 4 of those - I have never done so much on rotten window/door frames, failing DPC etc since moving here.
    • CommentAuthornikhoward
    • CommentTimeSep 29th 2012 edited
     
    What I have done on one of my walls if it helps:

    Removed all failing (and ugly) pebbledash, cut lime in render, chiseled off all render above and below DPC, cut French drain, removed interior plaster and render, tanked inside wall (slurry), new waterproof render/plaster, new DPM, floating floor.
      image.jpg
    • CommentAuthornikhoward
    • CommentTimeSep 29th 2012
     
    Will be EWI this wall soon 200mm EPS, acrylic render with mesh.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 29th 2012
     
    Posted By: nikhowardRender will wick up moisture
    Depends on its make-up - whether the mix of aggregate sizes filled all the voids (hopefully) or instead left a network of tiny voids/passages. Like I said,
    Posted By: fostertom... the more consistent, and narrow gap/passages within a slab of material

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