Since the material properties of the brick are so different from mortar and regular brick, fire brick is usually installed to be independent of the regular masonry construction.
Measuring the Heat Retention of Fire Brick
The measure of the heat retention of a material is composed of several different properties, which are evaluated separately. The thermal conductivity, heat capacity and density all contribute to the value of the heat retention of a material. The sum of these properties is the energy density of the material.
Thermal Conductivity of Fire Brick
Fire brick is typically used as an insulating material. Even so, ordinary dense fire brick has a fairly high thermal conductivity. The thermal conductivity of dense fire brick used to line fireplaces is 1.40 Watts per meter Kelvin (W/(m_K)). There are low density fire bricks used to line areas not subject to impact, such as furnaces. These fire bricks, composed of a silica ceramic, have a thermal conductivity as low as 0.24 W/(m_K).
Density of Fire Brick
There are two main types of fire brick. The dense type is typically used to line fireplaces and fireboxes of stoves that get a lot of abrasion from materials being burned. The light type of brick is mostly used as an insulator in places where it is not subject to mechanical abuse.
Dense fire brick has a density relative to water of 2.4, while light fire brick has a relative density of only 0.60. Since density is how much a material weighs compared to other materials of the same size, we use water as the basis of comparison for all materials.
Heat Retention of Fire Brick
The specific heat of a material is the amount of energy it takes to heat one kilogram of the material one degree centigrade, measured in Kelvin. The energy is measured in Joules. The amount of energy that is stored in fire brick is then a function of the density of the brick multiplied by the specific heat of the brick material.
A dense fire brick has an energy density of 2.52 kilojoules per cubic meter per degree Kelvin [kJ/(m3_K]. A light fire brick has an energy density of only 0.36 kJ/(m3_K). By comparison, an ordinary red brick has an energy density of 1.4 kJ/(m3*K).
In summary, a dense fire brick stores about eight times as much heat energy as a light fire brick, and can transmit the heat to a cooler surface about five times faster.