NOTE: DIAGRAMS & ILLUSTRATIONS ARE NOT TO SCALE.
21
Figure 53
A
B
D
C
C
Hearth Extension Dimensions
A
16"
B
35"
C
8"
D
52"
Wall Shield Required
Where Less Than 12"
A
B
C
C
30"
24"
D
Methods of Determining 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 fi replace,
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 5
.
Using the k formula:
Minimum
k-value (per Inch) of
Specifi ed min.
thickness of
=
alternate material (
k
M
) x
thickness
alternate
k-value (per inch)
of listed
material (
T
M
)
of listed material (
k
L
)
material (
T
L
)
T
M
(inches) =
k
M
x
T
L
*.84
T
M
(inches) = *.35
x
1"
*.84
.416 (inches)=
.416
x
1"
Answer - The minimum required thickness of the Micore 160 is .417," there-
fore round up to nearest standard thickness available which is 1/2."
Minimum
r-value (per Inch)
Specifi ed 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) = *1.19
x
1"
* 2.86
.417 (inches)=
.416
x
1"
Answer - The minimum required thickness of the Micore 160 is .417," there-
fore round up to nearest standard thickness available which is 1/2."
Using the r formula:
Note: To convert inches to millimeters divide by .03937.
Listed Material
k (per inch)
r (per inch)
Listed Min.Thickness
Listed Material
.84
K
L
1.19
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
.56" (1/2")
Micore 160™
U.S. Gypsum
.35
2.86
.42" (3/8")
Micore 300™
U.S. Gypsum
.46
2.17
.55" (1/2")
Durock™ Cement Board
U.S. Gypsum
1.92
.52
2.29" (2-1/4")
Hardibacker™
1.95
.513
2.32" (2-3/8")
Hardibacker 500™
2.30
.435
2.74" (2-3/4")
Cultered Stone Hearth-
stone™
2.82
.355
3.36" (3-3/8")
Wonderboard
3.23
.31
3.85" (3-7/8")
Super Firetemp M
Johns-Manville
.61
1.64
.73" (3/4")
Super Firetemp L
Johns-Manville
.54
1.85
.64" (5/8")
Face brick
9.00
.111
10.71" (10-3/4")
Common brick
5.00
.20
5.95" (6")
Cement mortar
5.00
.20
5.95" (6")
Ceramic tile
12.5
.08
14.88" (14-7/8")
Marble
~11
~.09
13.10" (13-1/8")
Table 5
** If the hearth extension material(s) that is intended to be used is NOT listed
on Table 5, 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 specifi ed in the NFI Certifi cation manuals).
Note: Also see NFI (National Fireplace Institute) Certifi cation Manuals showing
other exceptable caluculation methods and acceptable alternate materials
which can be used.
