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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
1.28
T
M
(inches) = 0.43* x 1"
1.28
Answer using k = 0.34 x 1" = 0.34 = ³⁄₈"
³⁄₈" 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) = 0.78 x T
L
r
M
T
M
(inches) = 0.78 x 1"
2.33*
Answer using r = 0.34 x 1" = .034 = 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.
Hearth Extension Dimensions
A 16"
B 33"
C 8"
D 49"
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 1.28 0.78 1"
Alternative Values Min. Thick
Materials k r T
M
Wonderboard 1.92 0.56 1 ¹⁄₂"
Common brick 5.00 0.20 3 ⁷⁄₈"
Cement mortar 5.00 0.20 3 ⁷⁄₈"
Ceramic tile 12.5 0.08 9 ³⁄₄"
Marble 11.0 0.09 8 ⁵⁄₈"
Micore CV230 0.43 2.33 ³⁄₈"
(U.S. Gypsum)
Ceraform 126 0.27 3.70 ¹⁄₄"
(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
= 0.78 x 1" = 0.78.
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
Wall Shield Required
Where Less Than 12"
A
B
C
C
30"
24"
D
A wall shield is required where a continuous
perpendicular side wall is within 12" of the
fireplace opening, (
Figure 49
). Use a 24" W x
30" H wall shield constructed of millboard or a
durable, noncombustible material having an
equal or greater insulating value than K = .54BTU/
IN FT
2
HR °F. At no time may a perpendicular
side wall be closer than 8".
If fireplace is installed diagonally across a 90°
corner; no wall shields are required.