breathability of sand / cement render when it has lime in

18 Nov 2012
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United Kingdom
I am going to remove blown concrete render from a wall, and rerender with a well known proprietary silicone-based render. It is essential that it be breathable.
I have been advised by the contractor that the mixture:

4 sand : 1 lime : 0.5 cement

(or possibly 1 lime : 1 cement)

will be as breathable as the silicone-based render. Does anyone have any knowledge or advice on this please ?

I have also heard that mixing cement and lime increases the 'capilliarity' which is not good because it would actually promote water penetration ?
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The main contractor should specify what he or she requires. That spec. is, presumably, what you are quoting for?
However, forget the mix for the moment, you must go by the finish coat Mfr's Instructions. Read the bag and/or call them.
I would have preferred Natural Hydraulic Lime, and that was the original plan. However I have been informed that the cost would be 100 £ / m2 assuming three coats. That would make the cost of rendering that relatively small area, 6 foot by 25 foot, £1400 !
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I don't see how silicone can be breathable.
It does eeem surprising that a silicone based render is breathable. Krend do strongly market it as breathable though, and they have independent (supposedly) scientific reports on their website that support it.

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