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Hearth Extension Dimensions
A
B
C
D
Note: To convert inches to millimeters
divide by 0.03937
Figure 47
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
= k value per inch of alternate material
M
r
= r value per inch of alternate material
M
= minimum thickness required for alternate material
T
M
= standard thickness of the alternate material
T
S
k
= k value per inch of listed material
L
r
= r value per inch of listed material
L
= minimum thickness of listed material
T
L
Note: An asterix "*" indicates, it is a value taken from Table 1 .
Using the k formula:
Minimum
k-value (per Inch) of
thickness of
=
alternate material (k
alternate
k-value (per inch)
material (T
)
of listed material (k
M
T
(inches)
=
k
M
M
*.84
T
(inches)
=
*.35
M
*.84
.416 (inches) =
.416
Answer - The minimum required thickness of the Micore 160 is .417",
therefore round up to nearest standard thickness available which is 1/2".
Using the r formula:
Minimum
r-value (per Inch)
thickness of
=
of listed material
alternate
r-value (per inch)
material
of alternate material
T
(inches)
=
r
L
M
r
M
T
(inches)
=
*1.19
M
*2.86
.416 (inches)=
.416
16"
35"
8"
51"
C
Specified min.
) x
thickness
M
of listed
)
material (T
M
x
T
L
x
1"
x
1"
Specified min.
x
thickness
of listed
material
x
T
L
x
1"
x
1"
NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
C
B
D
Answer - The minimum required thickness of the Micore 160 is .417",
therefore round up to nearest standard thickness available which is 1/2".
Listed Material
** Approved Alternate Materials for Floor/Hearth Protection
AlternativeMaterials
Kaowool M Board
Micore 160™
U.S. Gypsum
Micore 300™
U.S. Gypsum
Durock™ Cement Board
U.S. Gypsum
Hardibacker™
Hardibacker 500™
)
Cultered Stone Hearth-
L
stone™
Wonderboard
Super Firetemp M
Johns-Manville
Super Firetemp L
Johns-Manville
Face brick
Common brick
Cement mortar
Ceramic tile
Marble
Table 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.
A
Listed Material
k (per inch)
r (per inch)
.84
1.19
r
K
L
L
Thermal Values
(rounded to nearest 1/8 inch)
k (per inch)
r (per inch)
r
K
M
M
.47
2.13
.35
2.86
.46
2.17
1.92
.52
1.95
.513
2.30
.435
2.82
.355
3.23
.31
.61
1.64
.54
1.85
9.00
.111
5.00
.20
5.00
.20
12.5
.08
~11
~.09
Listed Min.Thickness
"
1
T
L
MinimumThickness
Min. Thickness
T
M
.56" (1/2")
.42" (3/8")
.55" (1/2")
2.29" (2-1/4")
2.32" (2-3/8")
2.74" (2-3/4")
3.36" (3-3/8")
3.85" (3-7/8")
.73" (3/4")
.64" (5/8")
10.71" (10-3/4")
5.95" (6")
5.95" (6")
14.88" (14-7/8")
13.10" (13-1/8")
17
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