Expert: Bart Ogden - 3/19/2009

Question
I am having my fireplace rebuilt and the firebrick that was installed has lots of
efflorescence. The contractor grouted it solid to the old firebox shell. Our
chimney needs to be rebuilt but we are waiting for winter to end before we do
the work. I have read that a 1" airspace is good to have behind the firebrick.
The mason disagrees saying the brick will crack. How do we ensure that the
efflorescence will not return when he replaces the firebrick.

Answer
Hi Betsy,

This is from the Brick Institute of America Technical Note 19:

"Combustion Chamber. The shape and depth of the combustion chamber will greatly influence draft, combustion air requirements and the amount of heat reflected and radiated into the room. Figure 1 illustrates the shape and Table 1 provides recommended dimensions for the combustion chamber. These dimensions may be varied slightly, but the information given is based on successful designs. Significant changes should not be made without consulting a fireplace design consultant.

The sides and lower portion of the back of the combustion chamber should be vertical. Above the vertical portion of the back, the brick should be sloped forward towards the fireplace opening to support the metal damper and the clay flue lining. For the maximum amount of reflected heat into the room, the sloped portion of the back should be plane rather than concave. If it is concave, more heat will be reflected back into the fire rather than into the room. Greater splay of the sides also increases the amount of heat reflected into the room.

The combustion chamber should be constructed of nominal 4 in. (100 mm) thick brick. When refractory brick or firebrick are used, model building codes permit the total wall thickness to be reduced. Thin mortar joints, not more than 1/4 in.(6.4 mm), should be specified. A 1 in. (25 mm) air space should be provided between the combustion chamber wall and the backup wall, although not required by building codes. This air space provides for thermal expansion of the combustion chamber. A noncombustible, compressible, fibrous insulation or similar material should be wrapped around the combustion chamber to ensure that this air space is maintained. The backup wall should be no less than 4 in. (100 mm) in thickness around the back of the combustion chamber to support the loads from the smoke chamber and chimney above."

The combustion chamber should NOT be grouted solid to the backup wall.

If there is a chimney cap installed  and the chimney exterior is not leaking water, efflorescence should not return.  Immediately after the firebox is replaced, it will likely have an efflorescence "bloom".  This efflorescence can be simply brushed off and it should not return if water is not penetrating the structure.

Thanks for using AllExperts.Com and good luck with your project.

Sincerely,
A. Bart Ogden, President
Home Safe Hearth & Chimney, Inc.
Wichita, KS
Voice 316-265-9828
Fax 316-265-9890
www.HomeSafeKS.com

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