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polygonum

On 04/11/2012 09:03, wrote:

The gradual movement of water vapour is from interior to exterior,
since interior RH is higher on average. Thus evaporation of water
from the exterior of the wall is necessary to avoid dampness. You
mentioned a black paint, if you mean bitumen on the exterior then
this can gradually cause damp problems by preventing evaporation.
Painted cement render can occasionally too in walls that are
borderline in terms of how they handle damp, but that's not likely to
apply to a cavity wall. Are you sure they're cavity walls, as
everything you describe is a lot more likely to occur with non-cavity
walls?

NT

RH is all about water vapour.

If the interior water vapour is going through the wall then there is no
evaporation at the outside.

If that water vapour is condensing in or on the wall, that is a
different matter.

--
Rod