A modern masonry wood-burning fireplace with a factory built metal Class-A chimney system
Many homes that are (or were) heated with wood or fuel oil, or back in the day, coal, have (or had) masonry chimneys to move the exhaust from burning those fuels to the outside. The chases used to hide these chimney systems are often areas of very high air leakage into and out of a home. With the advent of modern heating equipment that more efficiently burns natural gas and propane, or a system that doesn’t use any burning fossil fuels inside the home to produce heat, such as electricity, the old masonry chimneys have all but gone by the wayside. 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 new homes being built today still have chimney systems, the masonry chimney is rare, most install metal chimneys or use PVC vents.
Common chimney systems and clearance to combustibles
All masonry and metal chimney systems require some sort of space between the chimney used to exhaust burned fuels and any wood framing or insulation. This space is required to prevent a build-up of heat and a potential fire. The amount of space needed depends on the system used. For example, a B-vent, which is a double wall metal vent commonly used for lower efficiency fossil fuel space and water heating requires a 1-inch clearance from any combustible surface. A masonry chimney system and the metal factory built, double and triple wall Class-A chimney system, commonly used for wood burning appliances, will require a minimum 2-inch clearance to combustibles.
This B-Vent chimney venting the exhaust of a gas fireplace requires a minimum of 1-inch clearance to the wood framing.
Because these chimneys and flues can get hot, connection to the air control layer can be complicated, you cannot simply tape to the chimney, other products need to be used to seal to the air barrier.
A problem I see often
In my very cold climate, wood burning fireplaces and woodstoves are popular, especially in rural areas with an abundance of firewood. I occasionally see the traditional masonry style fireplace containing a concrete chimney with clay-lined flue, but it’s more common I see factory-built woodstoves and fireplaces using the Class-A double or triple wall chimney systems. The photos below show an all-too-common air leak. In the case of this home, a factory-built fireplace and metal chimney was set in a wood-framed chase. The chase had a poorly sealed air barrier between the framed chase and unconditioned vented attic. A blower door test showed cold air moving from the attic into the home.
The manufactured metal chimney systems have products designed to maintain the clearance to wood framing and keep insulation away from the chimney. Some systems have decorative ceiling support boxes for exposed stove pipe and chimney systems. Chimney systems can also be concealed, simple attic insulation shields and radiation shields are used to keep combustible materials away from hot chimney pipes. Though these shields maintain clearances, they are often not airtight. The use of a high temperature caulk is needed to reduce the infiltration and exfiltration of air around the chimney assembly. The caulk needs to be rated for the expected operating temperatures of the chimney system.
Illustrations by DuraVent, a manufacturer of chimney systems.
This B-vent radiation/insulation shield is designed to maintain the needed clearance from both the wood framing and any insulation installed around the chimney system.
Masonry chimney systems are more difficult to air seal than a manufactured system. The required space between the wood framing and chimney often runs the from the basement to the roof deck. This pathway can be a major mover of air, especially in cold climates with the prevalence of stack effect. How this air sealing is accomplished will depend on the access to the chimney space and the location of the air control layer. Ideally, we want the air control layer continuous. If the air control is located on the warm in winter side of the ceiling, this is the preferred location to perform the air sealing. Often, we do not have access to the warm side of a ceiling in a home, (masonry chimneys are often concealed inside finished walls), air sealing is then completed inside the attic, on the top side of the bottom cord of the rafter.
Typical masonry chimney as it enters the attic space. Two-inch clearance from framing.
Using a metal “L” flashing to span the two-inch gap between the concrete chimney and wood framing. Use a sealant or caulk rated for the expected temperatures to seal the metal to both the framing and chimney. Fasteners can be used to secure the metal flashing to the wood framing. Seal all four sides.
Add an insulation shield to maintain the two-inch clearance between the insulation and chimney. Do not install any insulation against a chimney, even if the insulation is rated for use in high temperature applications. The purpose of the air space is to dissipate the heat, keeping close, combustible surfaces cooler.
This method of air sealing would be for an active chimney, if the chimney is abandoned, clearances are no longer required, though there is always the chance a future homeowner will want to use the chimney. It might be a good idea to fix a permanent label informing future owners of the risks with a chimney that has been air sealed without maintaining the required clearance to combustibles.
Another way a fireplace or woodstove might vent exhaust is to a masonry chimney on the outside of the structure.
I’ve worked on a few homes where the masonry fireplace and chimney were constructed before the home was built. These can become air sealing nightmares, especially in very cold climates. Not only do you have a cold thermal mass migrating into the structure, but also possibly a clearance to combustible requirement that prevents the chimney to be in contact with framing. There are a few “it depends” caveats with this type of assembly, you may have to consult with an inspector and/or engineer to determine an approved method of air sealing when this condition exists.
The last type of chase I’ll discuss is called a wall pass through. This is where a stove pipe, the part of a chimney system that connects a wood burning appliance to a chimney system, passes through a wall and into a concealed or exterior chimney. A stove pipe is usually a single wall, 24 gauge or heavier blue or black steel. Stove pipes are not concealed inside a wall and usually become very hot. Because of these hot temperatures, special attention is required where they pass through a wall. There are two methods, the first is to purchase an approved UL listed factory-built wall pass through assembly. Much like a ceiling chimney system, a wall pass through will maintain the required clearance to framing and insulation. A second option is to build a pass through out of brick. The brick must extend out from the outer wall of the stove pipe 12-inches in all directions. In this case you’re having to deal with a roughly 24-inch by 24-inch brick wall built inside the wall cavity, displacing insulation and a stovepipe having very high temperatures pass through the brick. Very tough to air seal well. My advice, purchase the factory-built wall pass through. Again, you will need to use a high temperature caulk for the air sealing.
A through-wall system, illustration by DuraVent
Unsealed chimney chases can have many effects on a home. They affect heating and cooling costs, comfort, indoor air quality and can be a raceway for rodents and insects. Improper air sealing can result in a potential fire. Always maintain recommended clearances and use materials that will not burn when air sealing chimney chases.