A wall shield is required where a continuous perpendicular side wall is within 12" of the fireplace opening, (Figure 49 ). Use a 24" W x 30" H wall shield constructed of millboard or a durable, noncombustible material having an equal or greater insulating value than K = .54BTU/ IN FT2 HR ° F. At no time may a perpendicular side wall be closer than 8".

If fireplace is installed diagonally across a 90° corner; no wall shields are required.

C B C

A

D

Hearth Extension Dimensions

A

16"

 

 

B

33"

 

 

C

8"

 

 

D

49"

 

 

Figure 48

Wall Shield Required

Where Less Than 12"

24"

30"

C

B

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

 

 

 

 

 

 

Material

k value of listed

 

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

 

 

Material

r value of desired

 

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 combina-

“R value” = 1 = r x thickness of material used k

Alternative Hearth Extension

and Wall Shield Materials

Listed

Values

Min. Thick

 

 

 

 

Material

k

r

TL

Millboard

1.28

0.78

1"

 

 

 

 

Alternative

Values

Min. Thick

Materials

 

 

 

k

r

TM

Wonderboard

1.92

0.56

1 ¹⁄₂"

 

 

 

 

Common brick

5.00

0.20

3 ⁷⁄₈"

 

 

 

 

Cement mortar

5.00

0.20

3 ⁷⁄₈"

 

 

 

 

Ceramic tile

12.5

0.08

9 ³⁄₄"

 

 

 

 

Marble

11.0

0.09

8 ⁵⁄₈"

 

 

 

 

Micore CV230

0.43

2.33

³⁄₈"

(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

D

Figure 49

C A

tion 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.

NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.

*value taken from chart

17