Many homes that are (or were) heated with wood, oil, or—back in the day—coal have (or had) masonry chimneys to transport the exhaust made from burning those fuels to outside the house. The chases used to hide these chimney systems are often areas of high air leakage. With the advent of modern heating equipment that burns natural gas and propane more efficiently, and systems that don’t use fossil fuels to produce heat, masonry chimneys have all but disappeared. I have been in many homes, including my own, where the use of this type of chimney system has been discontinued. Mine no longer extends through the roof; it terminates in the attic space. Others have been completely removed. Some homes being built today have chimney systems but the masonry chimney is rare; most builders install metal chimneys or use PVC vents.

Clearance to combustibles

All chimney systems—whether masonry or metal—require some sort of space between the chimney and any wood framing or insulation. This space is required to prevent a build-up of heat and potential fire. The amount of space needed depends on the chimney system used. For example, a B-vent (a double-wall metal vent commonly used for lower efficiency, fossil-fueled space and water heating) requires a 1-in. clearance from any combustible surface. A masonry chimney system and factory-built metal double- and triple-wall Class A chimney systems—commonly used for wood-burning appliances—require a minimum 2-in. clearance to combustibles.

Metal chimney systems

In my…