Table of Contents
Advertisement
The hearth extension must extend beyond
both of the BM36ST fireplace side openings.
Use a hearth extension constructed of a du-
rable noncombustible material having an equal
or greater insulating value of k=.84 BTU IN/FT
HR °F or a thermal resistance that equals or
exceeds r=1.19 HR °F FT
³⁄₈" (10 mm) thick noncombustible material is
all that is required over a noncombustible or
slab floor ( Figure 44 ).
Secure the hearth extension to the floor to
prevent possible shifting.
Side Wall
12" (305 mm) Min.
to Fireplace Opening
C
B
D
C
Figure 44
C
B
D
Figure 45
Hearth Extension Dimensions
A
16" (406 mm)
B
26" (660 mm)
C
D
42" (1067 mm)
Note: Any noncombustible material whose k
value is less than .84 or whose r value is more
than 1.19 is acceptable.
16
If the fireplace is installed on a combustible
floor, use the metal safety strips (provided)
on the floor extending half under the fire-
place and half under the hearth extension
(refer to Figures 5 and 6 ).
2
If fireplace is installed diagonally across a 90°
2
/BTU IN. A minimum
corner; no wall shields are 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
k
M
r
M
T
L
Example: Micore CV230 is to be used for the
hearth extension fireplace. How thick must this
material be?
Using the k formula:
A
Desired
Required = material (per inch) x of Listed
Thickness k value of listed
T
(inches) = k
M
(inches) = 0.43* x 1"
T
M
C
A
Answer using k = 0.51 x 1" = 0.51 = ¹⁄₂"
¹⁄₂" thickness Micore will be required.
Using the r formula:
Desired
Required = material (per inch)x of Listed
Thickness r value of desired
T
(inches) = 1.19 x T
M
8" (203 mm)
T
(inches) = 1.19
M
Answer using r = 0.51 x 1" = .051 = ¹⁄₂"
¹⁄₂" thickness Micore will be required.
= Thickness of material in inches
= K value of desired material
= R value of desired material
= Minimum listed thickness
k value of desired
Min. thickness
Material
material (per inch)
x T
M
L
.84
0.84
r value of listed
Min. thickness
Material
material (per inch)
L
r
M
x 1"
2.33*
NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
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
k
Alternative Hearth Extension
and Wall Shield Materials
Listed
Values
Material
k
r
Millboard
.84
1.19
Alternative
Values
Materials
k
r
Wonderboard
1.92
0.56
Common brick
5.00
0.20
Cement mortar
5.00
0.20
Ceramic tile
12.5
0.08
Marble
11.0
0.09
Micore CV230
0.43
2.33
(U.S. Gypsum)
Ceraform 126
0.27
3.70
(Johns-Manville)
Example: Given that the required "R value" for
a suitable hearth extension used must be equal
to or greater than:
= 1.19 x 1" = 1.19.
"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:
= 0.20 x 3 ¹⁄₂" = .70
"R"
= r x T
M
M
Using ¹⁄₂" of mortar to set the brick, "R" for the
mortar is calculated as follows:
= 0.20 x ¹⁄₂" = .10
"R"
= r x T
M
M
*value taken from chart
Min. Thick
T
L
1"
Min. Thick
T
M
2 ¹⁄₄"
6"
6"
14 ⁷⁄₈"
13 ¹⁄₈"
¹⁄₂"
³⁄₈"
Advertisement
Table of Contents
