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Wall Shield Required
Where Less Than 12"
24"
30"
C
B
D
C
Figure 46
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
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
Required = material (per inch) x of Listed
Thickness k value of listed
material (per inch)
T
(inches) = k
x T
M
M
L
0.84
(inches) = 0.43* x ⁵⁄₈"
T
M
0.84
Answer using k = 0.51 x ⁵⁄₈" = 0.32 = ³⁄₈"
³⁄₈" thickness Micore will be required.
Using the r formula:
Desired
r value of listed
Required = material (per inch)x of Listed
Thickness r value of desired
material (per inch)
T
(inches) = 1.19 x T
M
L
r
M
x ⁵⁄₈"
T
(inches) = 1.19
M
2.33*
Answer using r = 0.51 x ⁵⁄₈" = 0.32 = ³⁄₈"
³⁄₈" 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.
"R value" = 1 = r x thickness of material used
Alternative Hearth Extension
and Wall Shield Materials
A
Listed
Material
Millboard
Alternative
Materials
Wonderboard
Common brick
Cement mortar
Ceramic tile
Marble
Micore CV230
(U.S. Gypsum)
Ceraform 126
(Johns-Manville)
Min. thickness
Example: Given that the required "R value" for
Material
a suitable hearth extension used must be equal
to or greater than:
"R" = r x T
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 ¹⁄₂"
dimension is the height, "R" for the common
brick becomes:
Min. thickness
"R"
= r x T
M
Using ¹⁄₂" of mortar to set the brick, "R" for the
Material
mortar is calculated as follows:
"R"
= r x T
M
*
value taken from chart
NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
k
Values
Min. Thick
k
r
T
L
⁵⁄₈"
.84
1.19
Values
Min. Thick
k
r
T
M
1 ¹⁄₂"
1.92
0.52
3 ³⁄₄"
5.00
0.20
3 ³⁄₄"
5.00
0.20
9 ¹⁄₄"
12.5
0.08
8 ¹⁄₄"
11.0
0.09
³⁄₈"
0.43
2.33
¹⁄₄"
0.27
3.70
= 1.19 x ⁵⁄₈" = 0.74.
= 0.20 x 3 ¹⁄₂" = 0.70
M
= 0.20 x ¹⁄₂" = 0.10
M
A ³⁄₄" marble slab set in ¹⁄₂" mortar covers the
brick, "R" for the marble and mortar becomes:
= 0.09 x ³⁄₄" = .068
"R"
= r x T
M
M
= 0.20 x ¹⁄₂" = .10
"R"
= r x T
M
M
The sum of all "R values" is:
.70 + .10 +. 068 + .10 = .968
This would be an acceptable combination of
material for the hearth extension since the
total calculated "R value" of the materials used
exceeds the required "R value" of 0.74.
WARNING: THE CRACK BETWEEN THE
FIREPLACE AND THE HEARTH EXTEN-
SION MUST BE SEALED WITH A NON-
COMBUSTIBLE MATERIAL.
WARNING: WHEN INSTALLING THE
HEARTH EXTENSION BE CAREFUL NOT
TO BLOCK THE HEAT-CIRCULATING AIR
INLET GRILL ON CIRCULATING MODELS.
WARNING: WHEN INSTALLING HEARTH
EXTENSION IN FRONT OF A CIRCULAT-
ING FIREPLACE MODEL, THE FIREPLACE
MUST BE RAISED IF HEIGHT OF HEARTH
EXTENSION EXCEEDS 2" ABOVE BOTTOM
OF FIREPLACE ( FIGURE 47 ).
5 1/2"
2"
Max. Thickness Of Hearth Extension When
Circulating Models Are On The Floor
Figure 47
Secure the hearth extension to the floor to
prevent possible shifting.
17
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