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NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
19
If there is a continuous perpendicular side wall
closer than 18" (mm) from the CST-38 fireplace
opening, it must be protected with a 40 x 40" x
¹⁄₂" (1016 mmx 1016 mm x 13 mm) wall shield
constructed of millboard or a durable noncom-
bustible material with an equal or greater insu-
lating value than k = .84 BTU IN/FT
2
HR °F or a
thermal resistance that equals or exceeds r =
1.19 HR °F FT
2
/BTU IN. A continuous side wall
can not be closer than 7" (178 mm) from the
nearest side of the CST-38 fireplace.
Secure the hearth extension to the floor to
prevent possible shifting.
A continuous perpendicular combustible side
wall can not be closer than 7" (178 mm) from
the nearest side of the CPF-38 fireplace. A wall
shield is not required.
Note: Any noncombustible material whose k
value is less than .84 or whose r value is more
than 1.19 is acceptable.
Hearth Extension Dimensions
CST-38 CPF-38
A 16" 16"
(406 mm) (406 mm)
B 28 ¹⁄₂" 32 ¹⁄₂"
(724 mm) (826 mm)
C 8" 8"
(203 mm) (203 mm)
D 44 ¹⁄₂" 48 ¹⁄₂"
(1130 mm) (1232 mm)
Figure 56
A
B
D
C
C
Figure 57
Figure 58
B
A
C
C
8" (203 mm) Min.
to Fireplace Opening
Side Wall
Model CST-38
D
B
A
C
C
25 ³⁄₁₆"
Optional
End Opening
Hearth
Extension
8" (203 mm) Min.
to Fireplace Opening
Side Wall
Model CPF-38
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*
Answer using r = 0.51 x 1" = .051 = ¹⁄₂"
¹⁄₂" thickness Micore will be required.
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.
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)