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12"
(305mm)
Max.
Combustible
Mantel
and Trim
12"
(305mm)
Min.
6
"
(167mm)
Typical Installation
Figure 46
Hearth Extensions and Wall Shields
A hearth extension must be installed with
all fireplaces. It is to protect the combustible
floor in front of the fireplace from both
radiant heat and sparks. The hearth exten-
sion must extend beyond the front at
least 20" (508 mm) and both sides at least
12" (305 mm) ( Figure 47 ). Use a hearth
extension constructed of a durable noncom-
bustible material having an equal or better
(lower k value) insulating value of k = .84
BTU IN/FT
2
HR F or a thermal resistance
that equals or exceeds r = 1.19 HR F FT
BTU IN. With these values, determine the
minimum thickness/material required using
the formula and Page 17 .
Note: Any noncombustible material whose 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, use the metal safety strips (provided)
on the floor extending half under the fire-
place and half under the hearth extension.
A wall shield is required where a continuous
perpendicular side wall is within 12" of the
fireplace opening ( Figure 48 ) . Use a 36" W x
36" H wall shield constructed of a durable,
noncombustible material having an equal or
better (lower k value) insulating value than k =
.54 BTU IN/FT
2
HR F. At no time may a
perpendicular side wall be located closer than
9" from the fireplace opening.
If fireplace is installed diagonally across a 90
corner; no wall shields are required.
16
Finished
Wall
1
" (38mm)
Noncombustible
Wall Covering
Spacer
Fireplace
Opening
Figure 47
2
/
NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
Hearth Extension Dimensions
A
20" (508 mm)
B
36" (914 mm)
C
12" (305 mm)
D
60" (1524 mm)
Methods of Determini
ng Hearth Extension and Wall Shield
Equivalents - To determine the thickness required for the alternate
material when either the "k" value or "r" value is known, use either the
k formula or r formula.
Example: If Micore 160 is to be used for the hearth extension fireplace,
how thick must this material be?
k
= k value per inch of alternate material
M
r
= r value per inch of alternate material
M
= minimum thickness required for alternate material
T
M
= standard thickness of the alternate material
T
S
k
= k value per inch of listed material
L
r
= r value per inch of listed material
L
T
= minimum thickness of listed material
L
Note: An asterix "*" indicates, it is a value taken from Table 1 .
Using the k formula:
Minimum
k-value (per Inch) of
thickness of
=
alternate material (k
alternate
k-value (per inch)
material (T
)
of listed material (k
M
T
(inches)
=
M
T
(inches)
=
M
.416 (inches) =
Answer - The minimum required thickness of the Micore 160 is .417",
therefore round up to nearest standard thickness available which is 1/2".
Using the r formula:
Minimum
r-value (per Inch)
thickness of
=
of listed material
alternate
r-value (per inch)
material
of alternate material
T
(inches)
=
M
T
(inches)
=
M
.416 (inches)=
C
B
D
Specified min.
) x
thickness
M
of listed
)
material (T
M
k
x
T
M
L
*.84
*.35
x
1"
*.84
.416
x
1"
Specified min.
x
thickness
of listed
material
r
x
T
L
L
r
M
*1.19
x
1"
*2.86
.416
x
1"
C
A
)
L
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