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NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
17
Figure 48
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
M
= Thickness of material in inches
k
M
= K value of desired material
r
M
= R value of desired material
T
L
= Minimum listed thickness
Example: Micore CV230 is to be used for the
hearth extension fireplace. How thick must this
material be?
Using the k formula:
Desired k value of desired Min. thickness
Required = material (per inch) x of Listed
Thickness k value of listed Material
material (per inch)
T
M
(inches) = k
M
x T
L
.43
T
M
(inches) = 0.43* x 1"
0.43
Answer using k = 1 x 1" = 1
1" thickness Micore will be required.
Using the r formula:
Desired r value of listed Min. thickness
Required = material (per inch)x of Listed
Thickness r value of desired Material
material (per inch)
T
M
(inches) = 2.33 x T
L
r
M
T
M
(inches) = 2.33 x 1"
2.33*
Answer using r = 1 x 1" = 1 =
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.
Hearth Extension Dimensions
A 16"
B 35"
C 8"
D 51"
A
B
D
C
C
“R value” = 1 = r x thickness of material used
k
Alternative Hearth Extension
and Wall Shield Materials
Listed Values Min. Thick
Material k r T
L
Millboard .43 2.33 1"
Alternative Values Min. Thick
Materials k r T
M
Wonderboard 1.92 0.56 4 ¹⁄₄"
Common brick 5.00 0.20 11 ⁵⁄₈"
Cement mortar 5.00 0.20 11 ⁵⁄₈"
Ceramic tile 12.5 0.08 29 ¹⁄₈ "
Marble 11.0 0.09 25 ⁷⁄₈"
Micore CV230 0.43 2.33 1"
(U.S. Gypsum)
Ceraform 126 0.27 3.70 ⁵⁄₈"
(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
L
= 2.33 x 1" = 2.33.
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 ¹⁄₂"
dimension is the height, “R” for the common
brick becomes:
“R”
M
= r x T
M
= 0.20 x 3 ¹⁄₂" = .70
Using ¹⁄₂" of mortar to set the brick, “R” for the
mortar is calculated as follows:
“R”
M
= r x T
M
= 0.20 x ¹⁄₂" = .10
*value taken from chart
If fireplace is installed diagonally across a 90°
corner; no wall shields are required.
Wall Shield Required
Where Less Than 12"
A
B
C
C
36"
36"
D