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 extension must extend 12" beyond both sides of the fireplace opening and 20" beyond the front (Figure 43 ). Use a hearth extension constructed of a durable noncombustible material having an equal or greater insulating value of k = .27BTU/IN FT2 HR ° F or a thermal resistance that equals or exceeds r = 3.70 HR ° F FT2 IN/BTU. With these values, determine the minimum thickness/ material required using the formula and the Table on page 17.
Note: Any noncombustible material whose k value is less than .27 or whose r value is more than 3.70 is acceptable.
If the fireplace is installed on a combustible floor, use the metal safety strips (provided) on the floor extending half under the fireplace and half under the hearth extension (refer to Figures 6 and 7 ).
C 
B 
C 
A
D
Figure 43
Wall Shield Required
Where Less Than 15"
24” 
36”
C
B
Answer using k = 1.59 x 1" = 1.59
1 ¹⁄₂" thickness Micore will be required.
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) |
| |
TM (inches) | = | 3.70 |
| x TL | |
|
|
| rM |
| |
TM (inches) | = |
| 3.70 |
| x 1" |
|
| 2.33* |
| ||
Answer using r = 1.59 x 1" = 1.59
1 ¹⁄₂" thickness Micore will be required.
At times it is important to know what combination of materials are acceptable for use as hearth extensions. The “R values” are used to determine acceptable combinations of materials because “R values” are additive where r and k values are not.
“R value” = 1 = r x thickness of material used k
Wall shields are required where a continuous perpendicular side wall is within the limits specified below. Use a wall shield constructed of 1/2" millboard or other durable, noncom- bustible material having an equal or greater
D
Figure 44
C A
Alternative Hearth Extension and Wall Shield Materials
insulating K value than those detailed below.
Minimum clearance to an unshielded perpendicular sidewall for the LA41 fireplace is 20" (508mm), where an adjacent wall is closer, it must be shielded with materials developing a K factor of 0.54 or less. The wall shield must be a minimum of 24" W x 36" H, (Figure 44 ) at no time may a perpendicular sidewall be closer 14" (356mm) shielded or not.
If fireplace is installed diagonally across a 90° corner; no wall shields are required.
Hearth Extension Dimensions
(For Figures 43 and 44 )
A20"
B41"
C12"
D65"
16
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:
TM | = Thickness of material in inches | |
kM | = K value of desired material | |
rM | = | R value of desired material |
TL | = | 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) |
| |||||
TM (inches) | = |
| kM x | TL |
| ||
|
|
| .27 |
|
|
|
|
TM (inches) | = | 0.43* | x 1" |
| |||
|
| 0.27 |
|
|
| ||
NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
Listed | Values | Min. Thick | |
Material | k | r | TL |
Millboard | .43 | 2.33 | 1 ¹⁄₂" |
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Alternative | Values | Min. Thick | |
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Materials | k | r | TM |
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Wonderboard | 1.92 | 0.56 | 7" |
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Common brick | 5.00 | 0.20 | 18 ¹⁄₂" |
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Cement mortar | 5.00 | 0.20 | 18 ¹⁄₂" |
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Ceramic tile | 12.5 | 0.08 | 46 ¹⁄₄" |
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Marble | 11.0 | 0.09 | 40 ³⁄₄" |
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Micore CV230 | 0.43 | 2.33 | 1 ¹⁄₂" |
(U.S. Gypsum) |
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Ceraform 126 | 0.27 | 3.70 | 1" |
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*value taken from chart