On Friday, November 2, 2012 9:26:17 PM UTC, wrote:

Hello.
I have a 1940s house, so even though it has cavity walls (I'm still learning about all this) can cement rendering cause dampness on internal walls?
My kitchen and extension's external walls are rendering. One part is "hollow" though it was fixed and the dampness is more noticeable after rain. Also both rooms have a concrete floor.
Thank you.
Ed.


Hi all,
thanks for the replies.
The damp is in the inside of the external walls but also in the dividing wall.
It goes up about 4 feet.
The plaster seems to stop about an inch above the floor.
I say it's *in* the walls. The bricks are actually damp.
There's no radiator in the kitchen but I had one put in the extension. It seems to dry out the wall until it rains again.
It could be that the DPC has been bridged but I can't even see a DPC to be honest. The bricks were painted black. The paint had peeled about below where the patch of hollow render is.
Could it be a water course under the extension?
Thanks again all.


And to add, the render finishes about 2 feet above the ground and is curved outwards, which I assume is to stop rain dropping inward.


The most likely cause is condensation, caused primarily by too high an interior RH. This type of thing is often misdiagosed as rising damp, which although it exists, is unusual. The solution is normally to address interior sources of dampness, eg showers without adequate ventilation, hob cooking on excessively high heat, drying clothes indoors, unvented gas heating, inadequate ventilation in rooms etc.

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