NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
17
This would not be an acceptable combination
of material for the hearth extension since the
total calculated “R value” of the materials used
is under the required “R value” of 1.19. An
additional layer of insulating materials must
be used.
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.
Answer using r = 0.51 x
1
" = .051 =
¹⁄₂
"
¹⁄₂
" 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.
Alternative Hearth Extension
and Wall Shield Materials
Listed
Values
Min. Thick
Material
k
r
T
L
Millboard
.84
1.19
1
"
Alternative
Values
Min. Thick
Materials
k
r
T
M
Wonderboard
1.92
0.56
2
¹⁄₄
"
Common brick
5.00
0.20
6"
Cement mortar
5.00
0.20
6"
Ceramic tile
12.5
0.08
14
⁷⁄₈
"
Marble
11.0
0.09
13
¹⁄₈
"
Micore CV230
0.43
2.33
¹⁄₂
"
(U.S. Gypsum)
Ceraform 126
0.27
3.70
³⁄₈
"
(Johns-Manville)
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) = 1.19 x T
L
r
M
T
M
(inches) = 1.19
x
1
"
2.33*
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
= 1.19 x
1
" = 1.19.
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
A
³⁄₄
" marble slab set in
¹⁄₂
" mortar covers the
brick, “R” for the marble and mortar becomes:
WARNING: WHEN INSTALLING HEARTH
EXTENSION IN FRONT OF THE FIRE-
PLACE, THE FIREPLACE MUST BE RAISED
IF HEIGHT OF HEARTH EXTENSION EX-
CEEDS 3 5/16" ABOVE BOTTOM OF FIRE-
PLACE (
FIGURE 47 ).
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
.84
T
M
(inches) = 0.43* x
1
"
0.84
Answer using k = 0.51 x
1
" = 0.51 =
¹⁄₂
"
¹⁄₂
" thickness Micore will be required.
Figure 47
Secure the hearth extension to the floor to
prevent possible shifting.
3
³⁄₄
"
3
⁵⁄₁₆
"
Max. Thickness Of Hearth Extension When
Fireplace Is On The Floor
“R”
M
= r x T
M
= 0.09 x
³⁄₄
" = .068
“R”
M
= r x T
M
= 0.20 x
¹⁄₂
" = .10
The sum of all “R values” is:
.70 + .10 +. 068 + .10 = .968
“R value” = 1 = r x thickness of material used
k
