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3.6.1 Calculating R-Value
The below information demonstrates how to calculate the R-Value for a non-compliant N.F.P.A. 211 hearth
extension.
The R-
Value is a measurement of a material’s insulating properties with respect to a referenced thickness (see
Table 3-4). The total R-Value of the hearth extension constructed out of different materials can be calculated by
summing the R-Value of each material with respect to the thickness (see equation below).
Table 3-4: Thermal Properties of Materials
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Material
Thermal Resistance
[R-Value] per Inch
Thermal Conductivity
[k-Value] per Inch
Cement Board (Durock Next Gen)
0.780
1.282
Brick
0.200
5.000
Limestone
0.153
6.536
Slate
0.100
10.000
Concrete
0.095
10.526
Marble
0.090
11.111
Granite
0.083
12.048
Ceramic Tile
0.080
12.500
R
Total
=
(
R
1
×T
1
)
+
(
R
2
×T
2
)
+
(
R
3
×T
3
)
+…
Note the k-Value per inch can be converted to an R-Value per inch using the following formula:
R=
1
k
Example:
A hearth extension is constructed out of 4” of concrete with 14.5” brick. The total R-Value is:
R
Total
= (R
con
×T
con
)+ (R
brick
×T
brick
) = (0.095 × 4")+ (0.200 × 14.5") = 3.280
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Values based on manufacturers technical specifications
