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
