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Hearth Extensions and Wall Shields
A hearth extension must be installed with all
fireplaces. It is to protect the floor in front of the
fireplace from both radiant heat and sparks. For
Model EST-48, the hearth extension must ex-
tend 12" beyond both sides and must extend
20" beyond the front. Use a durable noncom-
bustible material having an equal or greater
insulating value of k = .84 BTU IN/FT
or a thermal resistance that equals or exceed
r = 1.19 HR F FT
2
/BTU IN. A minimum 1" thick
noncombustible material is all that is required
over a noncombustible slab floor.
Note: Any noncombustible material whose 1" k
value is less than .84 or whose r value is more
than 1.19 is acceptable.
If the fireplace is installed on a combustible
floor, the metal safety strips (provided) must be
used on the floor extending half under the
fireplace and half under the hearth extension.
Secure the hearth extension to the floor to
prevent possible shifting.
If the fireplace is installed diagonally across a
90 corner, the 24" sidewall clearance can be
reduced to 20" and no wall protection is re-
quired.
C
B
C
Figure 44
C
B
D
Figure 45
16
2
HR F
Methods of Determining Hearth Extension
Equivalents
To determine the thickness required for any
desired material when either the k or r values
are known:
T
= thickness of desired material in inches
M
k
= k value of desired material
M
r
= r value of desired material
M
T
= minimum listed thickness
1
Example: Micore CV230 is to be used with the
EST-48 fireplace. How thick must this material
be?
Using the k formula:
Desired
Required = material (per inch) x of listed
Thickness
Side Wall cannot be
T
(inches) = k
M
Closer than 24" under
any Circumastances
T
(inches) = 0.43* x 1" **
M
Answer using k = 0.51 x 1 = 0.51 =
Using the r formula:
A
Desired
Required = material (per inch) x of listed
Thickness
T
(inches) = 1.19 x T
M
T
(inches) = 1.19 x 1" **
M
Answer using r = 0.51 x 1" = 0.51 =
C
A
* value taken from chart
** minimum thickness per listing
NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
Hearth Extension Dimensions
(For Figures 44 and 45 )
A
20"
B
48"
C
12"
D
72"
k value of desired
Min. Thickness
k value of listed
material
material (per inch)
x T
M
1
.84
.84
"
k value of desired
Min. Thickness
k value of listed
material
material (per inch)
1
r
M
2.33*
"
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)
At times, it is important to know what combina-
tions of materials are acceptable for use as
hearth extension. 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
k
Example: Given that the required "R" value for
a suitable hearth extension used with the EST-48
must be equal to or greater than "R" = r x T
1.19 x 1" = 1.19. It is desired to elevate a marble
hearth extension to a level of 5" or more off the
floor surface. What combination of noncom-
bustible materials can be used to accomplish
this?
If common brick is used so that the 3
dimension is the height, then "R" for the com-
mon brick becomes:
"R"
= r x T
= 0.20 x 3
" = 0.70
M
M
Using " of mortar to set the brick, then "R" for
the mortar becomes:
"R" = r x T
= 0.20 x
" = 0.10
M
A
" marble slab set on
" micore covers the
brick, then "R" for the marble and micore
becomes:
"R" = r x T
= 0.09 x
" = 0.068
M
"R" = r x T
= 2.33 x
" = 0.5825
M
Min. Thick
T
L
1"
Min. Thick
T
M
2 "
6"
6"
15"
13"
"
"
=
1
"
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