NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
17
Figure 47
A
B
D
C
C
Hearth Extension Dimensions
A
16"
B
38"
C
8"
D
54"
Note: To convert inches to millimeters
divide by 0.03937
Answer - The minimum required thickness of the Micore 160 is .875”,
therefore round up to 7/8” and use the nearest standard thickness available
which is 1”.
** If the hearth extension material(s) that is intended to be used is NOT listed
on Table 1, the material can still be used if the material(s) is noncombustible.
However, the manufacturer of the material must provide either the listed k-
value per inch or r-value per inch with listed thickness so that the minimum
thickness required for the hearth can be calculated (per instructions on this
Page and/or as specified in the NFI Certification manuals).
Note: Also see NFI (National Fireplace Institute) Certification Manuals showing
other acceptable calculation methods and acceptable alternate materials
which can be used.
Listed Material
k (per inch)
r (per inch)
Listed Min.Thickness
Listed Material
0.43
K
L
2.33
r
L
1
"
T
L
** Approved Alternate Materials for Floor/Hearth Protection
AlternativeMaterials
Thermal Values
MinimumThickness
(rounded to nearest 1/8 inch)
k (per inch)
K
M
r (per inch)
r
M
Min. Thickness
T
M
Kaowool M Board
.47
2.13
1.09" (1-1/8")
Micore 160™
U.S. Gypsum
.35
2.86
.81" (7/8")
Micore 300™
U.S. Gypsum
.46
2.17
Durock™ Cement Board
U.S. Gypsum
1.92
.52
4.47" (4-1/2")
Hardibacker™
1.95
.513
4.53" (4-1/2")
Hardibacker 500™
2.30
.435
5.35" (5-3/8")
Cultered Stone Hearth-
stone™
2.82
.355
6.56" (6-1/2")
Wonderboard
3.23
.31
7.51" (7-1/2")
Super Firetemp M
Johns-Manville
.61
1.64
1.42" (1-3/8")
Super Firetemp L
Johns-Manville
.54
1.85
Face brick
9.00
.111
20.93" (20-7/8")
Common brick
5.00
.20
Cement mortar
5.00
.20
Ceramic tile
12.5
.08
Marble
11.0
.09
Table 1
1.07" (1-1/8")
1.26" (1-1/4")
11.63" (11-5/8")
11.63" (11-5/8")
29.07" (29-1/8")
25.58" (25-5/8")
Methods of Determini
ng Hearth Extension and Wall Shield
Equivalents - To determine the thickness required for the alternate
material when either the “k” value or “r” value is known, use either the
k formula or r formula.
Example: If Micore 160 is to be used for the hearth extension fireplace,
how thick must this material be?
k
M
= k value per inch of
alternate material
r
M
= r value per inch of
alternate material
T
M
= minimum thickness required for
alternate material
T
S
= standard thickness of the
alternate material
k
L
= k value per inch of
listed material
r
L
= r value per inch of
listed material
T
L
= minimum thickness of
listed material
Note: An asterix “*” indicates, it is a value taken from Table 1.
Using the k formula:
Minimum
k-value (per Inch) of
Specified min.
thickness of
=
alternate material (k
M
) x
thickness
alternate
k-value (per inch)
of listed
material (T
M
)
of listed material (k
M
)
material (T
L
)
T
M
(inches)
=
k
M
x
T
L
*.43
T
M
(inches)
=
*.35
x
1"
*.43
.875 (inches) =
.875
x
1"
Answer - The minimum required thickness of the Micore 160 is .875”,
therefore round up to 7/8” and use the nearest standard thickness
available which is 1”.
Using the r formula:
Minimum
r-value (per Inch)
Specified min.
thickness of
=
of listed material
x
thickness
alternate
r-value (per inch)
of listed
material
of alternate material
material
T
M
(inches)
=
r
L
x
T
L
r
M
T
M
(inches)
=
*2.33
x
1"
*2.86
.875 (inches) =
.875
x
1"
