How a Will County winter works on a Joliet chimney
Joliet gives a chimney no easy season, but winter is where the real damage is done. When a fire burns, the wood gives off moisture and unburned particles that rise up the flue, cool against the cold masonry, and condense into creosote, a tar-like residue that coats the inside of the liner. The colder the flue, the more of it sticks, and a flue running up the outside wall of an older canal-district home stays cold all winter. Layer enough creosote on the walls of that flue and you have the fuel for a chimney fire sitting directly over your firebox, which is exactly the situation a yearly sweep is meant to prevent.
The masonry has its own enemy, and it is water frozen at the wrong moment. Brick and mortar are porous, and over a Joliet year they soak up rain and snowmelt. When that trapped moisture freezes it expands, and the relentless freeze-thaw cycling of a Will County winter pries the brick face apart and crumbles the mortar joints a little more with every cold snap. A crown that has cracked or a cap that has gone missing lets even more water straight into the structure, and the damage that shows up as spalling brick and a leaning chimney in spring was usually set in motion by water that got in the previous fall. This is why we push so hard on sealing the top of the chimney before the cold arrives, while there is still time to keep water and ice from ever finding their way in.