vent at top of blocked off cimney

Hi, yes its important that the chimney stacks themselves are kept ventilated the best way you can do this now that they have been capped off is by installing air brick in replace of exisitng bricks within the stack.

My advice would be to install vent cowls where the pots used to be to allow the stack to breathe. Not fitting any vent or cowl(s) makes the stack sweat and causes damp (like you have described). Also inspect any flashing`s around the stack perimeter for damage/tears etc. Another alternative would be to remove the stack if it`s not in use hence removing the problem,this all depend`s where the stack is located/shared. Hope this help`s.

Your builder should have known, when he capped of the chimney its common knowledge to fit a couple of vents to the stack and vents inside the building. Best to get a roofer to fit the vents, check all the pointing and flashings. As Andrew said, you if stack is redundtant take it down below roof and tile/slate over.

without viewing the job it would be impossible to say what your problem is coulds be the flashing round the chimney, a loose tile, badly fitted underfelt hole in the capping, pointing on the chimney failing, or un vented chimney the longer you leave this the more problems the dampness will cause your best bet is post the job in the roofing section on this site get few roofers round to look at your problem and diagnoise the problem and costs