I have a problem with damp in a granite block, mortared chimney stack attached to a 1973 kit-built bungalow. The stack is ventilated at top & bottom, with a gas fire fitted inside, the concrete cap is in good order, as are the roof-tiles adjoining. When the Westerly gales come, the upper half of the stack gets wet and stays wet, to the extent that the chimney breast inside efflouresces through the gyproc plasterboard. This has been going on years. Any ideas?
