Smoke Chamber Parging
In your masonry fireplace there is an area called the smoke chamber. They are shaped like an upside down tunnel and are designed to move the hot gases and smoke from your fire box to your liner. In out of code fireplaces the bricks are “corbelled” or stacked like upside down steps. Exposed corbelling increases smoke turbulence and reduces the efficiency of the smoke chamber. This allows dangerous creosote and soot to build in the smoke chamber and flue liner. To protect the integrity and increase the efficiency of your smoke chamber, the corbelled areas and any gaps, cracks or jagged edges should be coated and parged smooth with an insulating high-temperature mortar such as Cerfractory Foam.
The standard from the National Fire association is below the diagram
National Fire Protection Association (NFPA) Standards Manual
211...Standard For Chimneys, Fireplaces, Vents, and Solid Fuel Burning Appliances 2010 Edition
220.127.116.11...The inner surfaces of the smoke chamber shall be parge coated smooth, with an insulating refractory mortar,…
R1001.8…..Corbelling of masonry units shall not leave unit cores exposed to the inside of the smoke chamber…. Firebrick shall be laid with medium duty refractory mortar conforming to ASTM C199...When the inside surface of the smoke chamber is formed by corbelled masonry, the inside surface shall be parged smooth.
Best Cincinnati Chimney will either spray apply the cerfractory foam or hand parge the product on. This will cause less creosote build up as well as a better drafting chimney.