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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorandyman99
    • CommentTimeDec 12th 2018 edited
     
    Installers are currently EWI and rendering my extension. The front wall of the existing house was just going to be rendered externally with me adding 50mm EPS internally. There is a possibility that the installers could add 40mm EWI to this wall before they render (I wont go into the reason why 40, but assume this is the max). So the wall buildup from outside to in would be:

    40mm EWI(EPS) | brick/block cavity filled | 50mm IWI (EPS)

    Is this OK?
  1.  
    Why not ask the installers to get their insulation supplier to do a condensation risk analysis?
    •  
      CommentAuthordjh
    • CommentTimeDec 12th 2018
     
    What Nick said, plus adding EWI will raise the temperature of the wall and so make condensation less likely. Is the render permeable or not? If it's permeable (e.g. a lime-based render) I wouldn't have any concerns, but if it is impermeable there would be more to worry about.
  2.  
    Any gale flowing through the cavity to negate the EWI?
    • CommentAuthorandyman99
    • CommentTimeDec 13th 2018
     
    Thanks everyone. We're using a Sto render product. It's a silicone based finish which is meant to be breathable, or more breathable than the acrylic variant. Internal wall is stripped back to brick and a little plaster that didn't fall off whilst removing multiple internal layers. The cavity is densely packed wherever I've come across it opened up. Looking again this morning, it's quite a small wall and actually largely window, so it would seem there is little to lose by getting them to do the EWI?
  3.  
    Condensation risk analysis ( the simple kind anyway) will often indicate there is already a 'problem' with any cavity wall. The outer leaf is out in the cold and so the CRA indicates that water vapour will permeate and diffuse through the inner leaf then condense at the outer leaf.

    This apparent contradiction shows that a) CRA is probably too simplistic and raises unecessary concerns, b) cavity walls are tolerant of some condensation that drains away down through the cavity, that's what it is there for!

    Get your CRA done 'with' and 'without' the EWI and likely it will show EWI is an improvement on the existing.
  4.  
    Posted By: WillInAberdeenThis apparent contradiction shows that a) CRA is probably too simplistic and raises unecessary concerns, b) cavity walls are tolerant of some condensation that drains away down through the cavity, that's what it is there for!

    If the cavity is densely packed as described then I would suggest that any condensation on the outer leaf would be held by the insulation and not drain down the inner face.
  5.  
    Yep, that's another contradiction with the whole condensation 'risk analysis' business.

    Retrofit CWI 'should' make the outer leaf even colder, while resticting the drainage and ventilation path down the cavity, so 'should' cause additional condensation risk, according to the theory. Yet millions of buildings have had successful CWI without mass reports of damp problems.

    If someone proposes EWI or IWI of the same building, all kinds of concerns about condensation will be raised..!
  6.  
    Likewise, there are very many buildings with retrofit CWI and cement render on the outside, which 'should' be a recipe for condensation in the outer leaf, but no-one is concerned.

    But if anyone proposes breathable insulation and render instead of the cement render, they are expected to worry about condensation!
    • CommentAuthorbhommels
    • CommentTimeDec 13th 2018
     
    Just curious: how do you mitigate the fire risk of using EPS as IWI? Cladding/fire retardant...?
    •  
      CommentAuthordjh
    • CommentTimeDec 13th 2018
     
    Posted By: bhommelsJust curious: how do you mitigate the fire risk of using EPS as IWI? Cladding/fire retardant...?

    EPS has fire retardant in it, and it is usual to cover it with plasterboard and/or skim. So not much risk of fire and if there is one, not much chance of it spreading.
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