This is a wild one. We often get questions from people who want to know why their bedroom wall is always cold under the window. It's particularly obvious with Georgian townhouses, that have big, thick stone window sills.
Actually, it is very simple. Again, the damp industry loses out - it is all about temperature. Stone sills can get damp, especially if you paint them. Gloss paint on a stone sill is a no-no. Ideally they should never be painted. If they are, it traps moisture into the stone, and as we all know, damp stone will transmit heat - and is thus cold.
So when somebody points out mould growing on the wall under the window, I check to see if there is a stone sill. The imaging camera makes it so easy to see - a dark line, corresponding with the back of the stone, and that's just where the mould grows.
If only people would stop clagging their window sills with paint, and clean it all off to nice old stone.
If you have mould - clean it off - but you will need to dry that sill too - and that means getting the paint off it.
Thermal image of cold stone sill
... and the wall as it looks, with mould..
another thermal image of a cold window sill...
.. with its corresponding mouldy wall...
