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NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
16
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
2
/
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.
Fireplace
Opening
Typical Installation
6 ⁹⁄₁₆"
(167mm)
12"
(305mm)
Min.
Combustible
Mantel
and Trim
Spacer
Noncombustible
Wall Covering
Finished
Wall
12"
(305mm)
Max.
1 ¹⁄₂" (38mm)
Figure 47
A
B
D
C
C
Hearth Extension Dimensions
A 20" (508 mm)
B 36" (914 mm)
C 12" (305 mm)
D 60" (1524 mm)
Methods of Determining 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
M
= k value per inch of
alternate
material
r
M
= r value per inch of
alternate
material
T
M
= minimum thickness required for
alternate
material
T
S
= standard thickness of the
alternate
material
k
L
= k value per inch of
listed
material
r
L
= r value per inch of
listed
material
T
L
= minimum thickness of
listed
material
Note:
An asterix “*” indicates, it is a value taken from
Table 1
.
Using the k formula:
Minimum k-value (per Inch) of Specified min.
thickness of = alternate material (k
M
) x thickness
alternate k-value (per inch) of listed
material (T
M
) of listed material (k
M
) material (T
L
)
T
M
(inches) = kM xT
L
*.84
T
M
(inches) = *.35 x 1"
*.84
.416 (inches) = .416 x 1"
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) Specified min.
thickness of = of listed material x thickness
alternate r-value (per inch) of listed
material of alternate material material
T
M
(inches) = rL xT
L
r
M
T
M
(inches) = *1.19 x 1"
*2.86
.416 (inches)= .416 x 1"