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Why is cement render harmful to Cob houses?

 
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Why is cement render harmful to Cob houses?
 
gardener
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Interesting point of view... but I'm not sure they have it right that drying is the problem. There are multi-story cob/adobe structures in north Africa that may endure a rainy season of some sort, but are mostly in desert-like conditions where I can't imagine that the walls don't get baked dry by the sun for months. Also, my rocket mass heater has a 6" layer of cob six feet high, gets baked all winter, and is as strong as when first built.

On the other hand, I have seen plenty of examples where the vapor-proof cement skin applied to the outside in a climate with heating load has stopped moisture from escaping and caused the outer layer to swell and peel off. The garden wall in the video looks like that situation, as the lowest part of the wall has failed, right where you would expect groundwater to have possibly migrated up into the core if the stone base wasn't isolating it well enough.

Also, even seasonal variations in moisture level could cause the problem, as the clay moves slightly while the cement does not move at all. Good structural cob can distribute the stress of a tiny bit of movement due to the straw reinforcing.

As a potter, I know that slightly damp clay ("leather-hard", with just enough water content that it hasn't turned color to the bone-dry state) is the toughest, strongest state, but cob walls definitely do not stay in that sort of condition.
 
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the issue is that cement based coverings do not allow moisture to pass through the wall. Also , known as' breathing'
If a lime based plaster is used the problem will not occur.
the problem is that any timber etc within the wall will rot, if a cement based plaster is used.
 
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