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The use of an R value is convenient when more than one material is going to be used in the
hearth extension to cover the combustible surface. This is because R values are additive,
whereas K values are not. To find the corresponding R factor to use for some selected materials,
please see Table Thermal characteristics of common floor protection materials in this
section.
There are two ways to calculate the R factor of the floor protection. First, by adding the R-values
of materials used, or by the conversion if the K factor and thickness of the floor protection are
given.
To calculate the total R factor from R factors of the materials used, simply add the R-values of
materials. If the result is equal to or greater than the R-value requirements, the combination is
acceptable. To know the R-values of some selected materials, see table Thermal
characteristics of common floor protection materials.
Required floor protection R of 1.00. Proposed materials: four inches of brick and one inch
of Durock
®
board
Four inches of brick (R = 4 x 0,2 = 0,8) plus 1 inch of Durock
®
(R = 1 x 0.52 = 0.52).
0.8 + 0.52 = 1.32.
This R value is larger than the required 1.00 and is therefore acceptable
In the case of a known K and thickness of alternative materials to be used in combination,
convert all K values to R by dividing the thickness of each material by its K value. Add the R
values of your proposed materials as shown in the previous example.
Example:
K value = 0.75
Thickness = 1
R value = Thickness/K = 1/0.75 = 1.33
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