The Importance of Parging Smoke Chambers

First, I want to take a moment to talk about smoke chambers.  For those that don’t know, a smoke chamber is the area located directly above the firebox.  A smoke chamber’s main function is to funnel the byproducts of combustion (smoke) into the flue, so that it may exit the home. 

Smoke Chamber Construction
According to the IRC (International Residential Code) book R1001.8 and NFPA 211 (National Fire Protection Association) section 11.2.1, smoke chambers should be constructed of solid masonry units, hollow masonry units grouted solid, stone, or concrete and must be at least 8 inches thick.  The height of the inside of the smoke chamber should be less than the width of the firebox opening.  The smoke chamber should have a slope of less than 45 degrees from vertical.  Any corbeling, which is defined as a reverse stair stepping of brick, is to be parged smooth using a refractory mortar meeting the ASTM C199 (American Society for Testing and Materials) standard with the ability to withstand temperatures up to 1800°F (983°C). 

Why Parge Smoke Chambers
We now know that the IRC and NFPA both require corbeled smoke chambers to be parged smooth, but let’s talk about why.  Every time you have a fire; the smoke tries to make its way up your flue to the outside.  If you have a smoke chamber with corbeled brick exposed, the corbels will slow this process down.  Every time the smoke passes over a corbeled brick, it will form a small eddy current impeding the smoke behind it from quickly leaving your system.  This increase in residence time will cause the smoke to begin to cool.  As the temperature of the smoke decreases, volatile compounds begin to condense on cooler surfaces, initiating the formation of creosote deposits.  Creosote has a very low flash point (the temperature at which it ignites with a spark) at 165°F (74°C).  It also has a comparatively low temperature, and it will ignite without a spark at temperatures at or above 451°F (233°C).  Firebox temperatures of a wood-burning fireplace can have a widely varied range of 600°F-1,200°F (315°C-649°C), making creosote in a smoke chamber very dangerous. 

In Conclusion
Parging (smoothing) smoke chambers is not only a requirement under current NFPA 211 and IRC codes, but also a critical best practice to enhance safety by minimizing creosote accumulation and reducing the risk of more severe hazards.