They should spec a thickness.
Technical Note: Conductivity
Cement backer board is an ideal material for a hearth pad because of its low “k”, or conductivity value. Conductivity is the ability to conduct heat. The lower the “K” factor, the less heat is conducted through the material. For example, Durock has a k-value of 1.92 per inch, as compared to the k-value of common brick, which is 5.00 per inch, or marble, which has a k-value of 15.00 to 20.00 per inch
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If the appliance is to be installed on a combustible floor or a combustible floor covering, it must be installed on a 1” thick non-combustible millboard floor protector or a durable equivalent, with a “R” factor of no less than “2.” The pad must be installed beneath the unit, extending 16” (U.S.) on the side equipped with a door, and 8” on all other sides. The pad must cover any horizontal chimney connector runs and extend 2” beyond each side.
Alternate Floor Protection:
An easy means of determining if a proposed alternate floor protector meets requirements is to follow this procedure:
1) Convert specification to R-value:
i R-value is given – no conversion is needed
ii k-factor is given with a required thickness (T) in inches: R = 1/k x T
iii C-factor is given: R = 1/C
2) Determine the R-value of the proposed alternate floor protector:
i Use the correct formula given in step 1 (above) to convert values not expressed as “R.”
ii For multiple layers, add R-values of each layer to determine overall R-value.
3) If the overall R-value of the system is greater than the R-value of the specified floor protector, the alternate is acceptable.
The specified floor protector should be ¾” thick material with a k-factor of 0.84. The proposed alternate is 4” brick with a C-factor of 1.25 over 1/8” mineral board with a k-factor of 0.29.
Step (a): Use formula above to convert specification to R-value. R = 1/k x T = 1/0.84 x .75 = 0.893
Step (b): Calculate R of proposed system.
4” brick of C = 1.25, therefore R brick = 1/C =1/1.25 = 0.80
1/8” mineral board of k = 0.29, therefore Rmin.bd. = 1/0.29 x 0.125 = 0.431
Total R = Rbrick + Rmineral board = 0.8 + 0.431 = 1.231
Step (c): Compare proposed system of R of 1.231 to specified R of 0.893. Since proposed system R is greater than required, the system is acceptable.