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If fireplace is installed diagonally across a 90
corner; no wall shields are required.
C
B
D
Hearth Extension Dimensions
A
B
C
D
Figure 48
Wall Shield Required
Where Less Than 12"
36"
36"
C
B
D
C
Figure 49
Methods of Determining Hearth Extension
and Wall Shield Equivalents
To determine the thickness required for any
material when either the k or r values are
known:
T
= Thickness of material in inches
M
k
= K value of desired material
M
r
= R value of desired material
M
T
= Minimum listed thickness
L
C
A
Example: Micore CV230 is to be used for the
hearth extension fireplace. How thick must this
material be?
Using the k formula:
16"
Desired
Required = material (per inch) x of Listed
35"
Thickness k value of listed
8"
51"
T
(inches) = k
M
T
(inches) = 0.43* x
M
Answer using k = 1 x " = 1
" thickness Micore will be required.
Using the r formula:
Desired
Required = material (per inch)x of Listed
Thickness r value of desired
T
(inches) = 2.33 x T
M
A
T
(inches) = 2.33
M
Answer using r = 1 x " = 1 =
" thickness Micore will be required.
At times it is important to know what combina-
tion of materials are acceptable for use as
hearth extensions. The "R values" are used to
determine acceptable combinations of materi-
als because "R values" are additive where r and
k values are not.
NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
k value of desired
Min. thickness
Material
material (per inch)
x T
M
L
.43
"
0.43
r value of listed
Min. thickness
Material
material (per inch)
L
r
M
x "
2.33*
"R value" = 1 = r x thickness of material used
k
Alternative Hearth Extension
and Wall Shield Materials
Listed
Values
Material
k
r
Millboard
.43
2.33
Alternative
Values
Materials
k
r
Wonderboard
1.92
0.56
Common brick
5.00
0.20
Cement mortar
5.00
0.20
Ceramic tile
12.5
0.08
Marble
11.0
0.09
Micore CV230
0.43
2.33
(U.S. Gypsum)
Ceraform 126
0.27
3.70
(Johns-Manville)
Example: Given that the required "R value" for
a suitable hearth extension used must be equal
to or greater than:
"R" = r x T
= 2.33 x " = 2.33.
L
If it is desired to elevate a marble hearth exten-
sion to a level of 5" or more above the floor
surface. What combination of noncombustible
materials can be used to accomplish this?
If common brick is used so that the 3
dimension is the height, "R" for the common
brick becomes:
"R"
= r x T
= 0.20 x 3
" = .70
M
M
Using " of mortar to set the brick, "R" for the
mortar is calculated as follows:
"R"
= r x T
= 0.20 x
" = .10
M
M
*value taken from chart
Min. Thick
T
L
"
Min. Thick
T
M
4 "
11 "
11 "
29 "
25 "
"
"
"
17
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