N. Thornton wrote in message ...
An open chimney is obviously going to let quite a bit of water come
in, I'd look into the possibility of one of those rain shedding cowls.
In a 9" wall you often only have 4" between the elements and the inside of
the chimney. I don't know how well soot retains moisture but it would help
explain why chimney breasts are often affected more than the adjacent wall.