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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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      CommentAuthorfostertom
    • CommentTimeJan 31st 2020
     
    Does anyone have any chapter and verse on cement renders?

    Cement render was a favourite way to 'waterproof' walls (little did they know back then that their water problem was prob condensation, not penetrating rain water).

    Anyway, by the 80s everyone had realised that cement render would actually make any damp problem worse. The render shrinks and would crack into few fairly large panels and the cracks would soak up driven rainwater sluicing across the impervious surface. Masonry walls are designed to absorb rain water - but then to evaporate it out again at the first opportunity. It turned out that the supposedly waterproof render was verey good at sucking in water through its capillary cracks, but that water then had v few routes by which to re-evaporate it out again. So water would build up in the wall.

    So the conventional wisdom reversed - hack the cement render off and replace it with breatheable lime render. That is what property surveyors routinely recommend today. Even better can be to simply repoint the exposed masonry properly with lime render.

    However, two things:

    First, cement render can often be fairly vapour permeable - depends on the mortar mix. You can't tell by visual inspection but a sample can be lab tested.

    Second, the particular case of rough-surfaced cement render - spatterdash ('tyrollean') or wet dash incorporating small shingle aggregate. As it shrinks, such rough-surfaced render, unlike smooth trowelled render, actually cracks into lots of micro-cracks, around each 'lump' of its surface, instead of just a few large cracks. These suck in rainwater just the same, but behave very differently during the re-evaporation phase. They in fact provide plentiful re-evaporation paths and the wall re-dries just fine. If rough-surfaced, even a render made of a thoroughly vapour-impermeable mortar mix will simulate a vapour-impermeable lime render.

    So, present conventional wisdom to remove cement renders ain't necessarily so. For example a well-adhered spatterdash cement render makes a fine base for application of EWI, allowing adhesive-only fixing without mechanical fixings, also providing the airtight layer, while being vapour-permeable.

    But not if it's been painted with vapour-impermeable plastic-based 'masonry paint'. Or if the spatterdash coat has been applied over an earlier smooth cement render, or if the spatterdash has been applied over an earlier plastic paint layer.

    So this is the question -

    First, any comments, wisdom, warnings, on any of the above?

    Second, any refs to chapter and verse, numbers even, on the veracity of the relative permeability of rough-surfaced vs smooth cement renders? And of paint layers?
    • CommentAuthorMike1
    • CommentTimeJan 31st 2020 edited
     
    I would certainly agree that hacking off cement renders and replacing them with lime shouldn't be automatic. It needs a proper analysis of the pros and cons relative to the particular situation and substrate.

    Worth noting too that while the favourite way of waterproofing walls may have been to use a render - and cement happened to be the go-to product post-war (cheap, strong, readily available and easy to use / less training required) - of course there were many other reasons for rendering buildings. For example as stucco to avoid the cost of real dressed stonework; to cover up brickwork (or old timber frame buildings) to be fashionable as stucco became popular; as a cheaper alternative to replacing spalled bricks; in recent years to cover external wall insulation; etc.

    The main problem I have with rough renders is that they tend to collect more dirt - and are also more difficult to clean and paint / limewash. And maybe the paint will be thicker, stopping up those micro-cracks? It would be interesting to read some research.
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      CommentAuthordjh
    • CommentTimeJan 31st 2020
     
    Posted By: Mike1The main problem I have with rough renders is that they tend to collect more dirt - and are also more difficult to clean and paint / limewash. A

    Hmm, dunno? We deliberately went with a rough surface for the alleged benefits of better drying, but it's not as rough as a dashed finish. I haven't noticed any problems with painting it, or rather I didn't hear any complaints from the people painting it :) I haven't even thought of any reason why I might want to clean it?
    • CommentAuthorMike1
    • CommentTimeJan 31st 2020
     
    Posted By: djhI haven't even thought of any reason why I might want to clean it?

    I guess that may depend where you live - probably more of an urban problem!
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