Parging

Above: A rough, unparged smoke chamber. Below: Same chamber after parging.

The smoke chamber is the area located directly above your fireplace damper. It is the passageway that funnels the smoke from the firebox into your chimney liner. Often, builders leave this area unfinished, with raw bricks and masonry and lots of little nooks and crannies.

This is less than optimal for three reasons:  first, building materials can be located just on the other side of the brick, which can lead to a potential fire hazard. A layer of high temperature mortar helps prevent heat from transferring through these bricks.

Second, a smooth smoke chamber transition will allow smoke to travel at a quicker rate up the chimney. A faster draft decreases the amount of creosote build up and decreases the chance of a chimney fire.

Third, creosote can collect in the crevices in raw masonry such that it cannot be removed by mechanical means.  This unsweepable creosote can ignite and start a chimney fire.

Parging the smoke chamber — covering it with a thick layer of mortar — helps insulate the chimney, reduces turbulence, and limits the number of places unsweepable creosote can accumulate.  It makes your chimney much more efficient to use and much easier to clean.