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

 

 

 

 

1.28

 

 

 

 

TM (inches)

=

0.43*

x 1"

 

 

 

1.28

 

 

 

Answer using k = 0.34 x 1" = 0.34 = ³⁄₈" ³⁄₈" 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)

=

0.78

 

 

x TL

 

 

 

 

rM

 

 

 

 

TM (inches)

=

 

0.78

 

x 1"

 

 

 

2.33*

 

 

 

Answer using r = 0.34 x 1" = .034 = ³⁄₈" ³⁄₈" 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

Alternative Hearth Extension Materials

Listed

Values

Min. Thick

Material

 

 

 

k

r

TL

Millboard

.84

1.19

³⁄₄"

 

 

 

 

Alternative

Values

Min. Thick

Materials

 

 

 

k

r

TM

Wonderboard

1.92

0.56

1 ³⁄₄"

 

 

 

 

Common brick

5.00

0.20

4 ¹⁄₂"

 

 

 

 

Cement mortar

5.00

0.20

4 ¹⁄₂"

 

 

 

 

Ceramic tile

12.5

0.08

11 ¹⁄₄"

 

 

 

 

Marble

11.0

0.09

10"

 

 

 

 

Micore CV230

0.43

2.33

¹⁄₂"

(U.S. Gypsum)

 

 

 

 

 

 

 

Ceraform 126

0.27

3.70

¹⁄₄"

(Johns-Manville)

 

 

 

 

 

 

 

Example: Given that the required “R value” for a suitable hearth extension used must be equal to or greater than:

“R” = r x TL = 0.78 x 1" = 0.78.

If it is desired to elevate a marble hearth extension 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:

“R”M = r x TM = 0.20 x 3 ¹⁄₂" = .70

Using ¹⁄₂" of mortar to set the brick, “R” for the mortar is calculated as follows:

“R”M = r x TM = 0.20 x ¹⁄₂" = .10

*value taken from chart

NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.

A ³⁄₄" marble slab set in ¹⁄₂" mortar covers the brick, “R” for the marble and mortar becomes:

“R”M = r x TM = 0.09 x ³⁄₄" = .068 “R”M = r x TM = 0.20 x ¹⁄₂" = .10

The sum of all “R values” is:

.70 + .10 +. 068 + .10 = .968

This would be an acceptable combination of material for the hearth extension since the total calculated “R value” of the materials used exceeds the required “R value” of 0.78.

WARNING: THE CRACK BETWEEN THE FIREPLACE AND THE HEARTH EXTENSION MUST BE SEALED WITH A NON- COMBUSTIBLE MATERIAL.

WARNING: WHEN INSTALLING HEARTH EXTENSION IN FRONT OF A NON-CIRCULATING FIREPLACE MODEL, THE FIREPLACE MUST BE RAISED IF HEIGHT OF HEARTH EXTENSION EXCEEDS 6" ABOVE THEBOTTOMOFFIREPLACE(FIGURE45 ).

6"

Max. Thickness Of Hearth Extension

When Models Are On The Floor

Figure 45

Secure the hearth extension to the floor to prevent possible shifting.

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