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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.

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    • CommentAuthorjfb
    • CommentTimeOct 29th 2018 edited
    I am trying to help out/advise friends who have bought an old, solid wall building in Wales and I thought it would be a prime candidate for ewi. Currently it is cement rendered so ewi wouldn't adversely affect its current look.
    Is it standard practise to hack off all cement render and then use a breathable ewi system? Or can you just fix straight onto the existing render (assuming it is sound enough) and go from there (much less work). Personally I it were mine I would hack it all off and repoint in lime if needed and go from there. What does anyone else think?
    I agree with your way. Many system providers will do it over whatever is there already, but that means you may have a 'breathable system' over a non-permeable substrate.
    • CommentTimeOct 29th 2018 edited
    It doesn't have to be all that breatheable - and it's a rare cement render that's really vapour impermeable. Usually it'll be a quantum leap more breatheable than a VCL.

    You can always drill lots of holes in it. Overall permeability isn't proportional to the permeable proportion of a wall face, it'll get permeable v fast with the first scattering of holes and adding more holes will be diminishing returns - I'd say a 6mm hole @ 200mm c/cs both ways.

    A well adhered flat (not necessarily smooth e.g. roughcast/spatterdash) face, incl any paint on it being well adhered, is a gift for EWI. Not only can you then fix simply by adhesive (subject to the on-site advice of the Rep of your chosen EWI products co), but sound render can be your airtight layer, if you don't destroy it.

    Note that airtightness and vapour tightness, though the same animal (water vapour is just one part of 'air'), have vastly different criteria for 'tightness'. What counts as amply 'tight' as airtightness is a much more leaky criterion than 'tight' for vapour tightness, which is an almost totally no-leakiness criterion. So you can say that a render is well airtight while still being adequately water-vapour permeable.

    But not of course airtight if you've drilled holes in it!
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