HearthStone Quality Home Heating Products, Inc ®
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WFP-75 Model 8411
R CALCULATIONS
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 1:
Thermal
Characteristics of Common Floor Protection Materials
table.
There are two ways to calculate the R factor of the floor protection. First, by adding the R-values of the
proposed materials or if some K and thickness values are given, by converting them to R values.
To calculate the R factor for a composite floor protection made of a combination of alternative materials,
simply add the R-values of those materials. If the result is equal to or larger than the required R value, the
combination is acceptable. For R-values of some selected materials, see
Table 1: Thermal
Characteristics of Common Floor Protection Materials.
Example: If the required floor protection R is equal or higher than 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) + 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.
In the previous example: Four inches of brick (K = 4/5 = 0.8) + 1 inch of Durock® (R = 1 / 1.92 = 0.52). 0,8
+ 0,52 = 1,32.
Table 1: Thermal Characteristics of Common Floor Protection Materials*
MATERIAL
CONDUCTIVITY (k) PER
INCH
RESISTANCE (R) PER INCH THICKNESS
Micore
®
160
0.39
2.54
Micore
®
300
0.49
2.06
Durock
®
1.92
0.52
Hardibacker
®
1.95
0.51
Hardibacker
®
500
2.3
0.44
Wonderboard
®
3.23
0.31
Cement mortar
5.00
0.2
Common brick
5.00
0.2
Face brick
9.00
0.11
Marble
14.3 – 20.00
0.07 – 0.05
Ceramic tile
12.5
0.008
Concrete
1.050
0.950
Mineral wool insulation
0.320
3.120
Limestone
6.5
0.153
Ceramic board (Fibremax)
0.450
2.2
* Information as reported by manufacturers and other resources.
