I have a damp spot in the centre of a wall that appears after rain. The spot appears about 5 ft up and abut 20 cm from the corner. The wall is the original back wall of the house but it is now an internal wall due to an extension. I'm in a semi and it's on the attached side, there is a narrow gap (too small for a person) between my extension and my neighbour's one.
I've read about rusty wall ties and something bridging the cavity but in this case the other side of the wall is in my extension and does not have a damp patch.
This wall doesn't have cavity wall insulation.
Does anyone have any suggestions?
TIA