Hi all, I have a 1930's detached house. In one of the bedrooms in the corner of 2 external works there is black mould that has started to grow. It's only in the corner and has discoloured the paint to a pale white colour and black mould inbetween. Could this be due to an issue on the outside or is it likely to be a ventilation problem. It's the only place in the house this has happened.