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Building Science

Air-Sealing a Chimney Chase

Common chimney systems and tips for sealing this problematic area prone to air leakage

A masonry wood-burning fireplace with a factory-built metal Class A chimney system

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.

B-Vent chimney venting
This B-vent chimney venting the exhaust of a gas fireplace requires a minimum of 1-in. clearance to the wood framing. Because chimneys and flues get hot, connection to the air control layer can be complicated; you cannot simply tape to the chimney. Other products are needed to seal to the air barrier.

Metal chimney systems

In my…

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  1. user-1112710372 | | #1

    Are there any code or safety issues sealing with the high temp caulk between the metal double wall chimney and wood? The chimney installers are worried about that - granted they are way done the scale on professionally trained by what I can tell.

  2. Expert Member


    Even insulated double-wall chimneys have a clearance (typically 2") from any wood. The only parts that can be attached to the framing are the ceiling or cathedral support, and above that the roof flashing. The ceiling/cathedral support can be air-sealed with high heat caulk.

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