Recently, one of our customers asked us to investigate a chimney that was rebuilt just a few years prior by another company. Two of the three chimney flues are no longer used, so the contractor elected to seal both of these flues at the top of the chimney.
Because a chimney is not insulated like your house walls, it needs to breathe. Otherwise, air expands and contracts on the inside of the chimney and builds up moisture. This causes frost which, over time, cracks the bricks and creates mineral deposits on the outside of the chimney, called efflorescence.
This customer has a pretty new chimney. And now it needs to be rebuilt, due to the amount of deterioration on the inside.