If there is a continuous perpendicular side wall closer than 18" (mm) from the CST-38 fireplace opening, it must be protected with a 40 x 40" x ¹⁄₂" (1016 mmx 1016 mm x 13 mm) wall shield constructed of millboard or a durable noncom- bustible material with an equal or greater insulating value than k = .84 BTU IN/FT2 HR ° F or a thermal resistance that equals or exceeds r =

1.19HR ° F FT2/BTU IN. A continuous side wall can not be closer than 7" (178 mm) from the nearest side of the CST-38 fireplace.

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

A continuous perpendicular combustible side wall can not be closer than 7" (178 mm) from the nearest side of the CPF-38 fireplace. A wall shield is not required.

Note: Any noncombustible material whose k value is less than .84 or whose r value is more than 1.19 is acceptable.

C B C

A

D

Figure 56

Hearth Extension Dimensions

 

CST-38

CPF-38

 

 

 

A

16"

16"

 

(406 mm)

(406 mm)

 

 

 

B

28 ¹⁄₂"

32 ¹⁄₂"

 

(724 mm)

(826 mm)

 

 

 

C

8"

8"

 

(203 mm)

(203 mm)

 

 

 

D

44 ¹⁄₂"

48 ¹⁄₂"

 

(1130 mm)

(1232 mm)

 

 

 

Side Wall

8" (203 mm) Min.

to Fireplace Opening

C

B

D C A

Model CST-38

Figure 57

Side Wall

8" (203 mm) Min.

to Fireplace Opening

Optional

End Opening

Hearth

Extension

25 ³⁄₁₆"

C

B

C A

Model CPF-38

Figure 58

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)

 

NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.

TM (inches) = kM x TL

.84

TM (inches) = 0.43* x 1" 0.84

Answer using k = 0.51 x 1" = 0.51 = ¹⁄₂" 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)

=

1.19

 

x TL

 

 

 

 

 

rM

 

 

 

 

TM (inches)

=

 

1.19

 

x 1"

 

 

 

2.33*

 

 

 

Answer using r = 0.51 x 1" = .051 = ¹⁄₂" 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.

Alternative Hearth Extension

and Wall Shield Materials

Listed

Values

Min. Thick

 

 

 

 

Material

k

r

TL

 

 

 

 

Millboard

.84

1.19

1"

 

 

 

 

Alternative

Values

Min. Thick

 

 

 

 

Materials

k

r

TM

Wonderboard

1.92

0.56

2 ¹⁄₄"

 

 

 

 

Common brick

5.00

0.20

6"

 

 

 

 

Cement mortar

5.00

0.20

6"

 

 

 

 

Ceramic tile

12.5

0.08

14 ⁷⁄₈"

 

 

 

 

Marble

11.0

0.09

13 ¹⁄₈"

 

 

 

 

Micore CV230

0.43

2.33

¹⁄₂"

(U.S. Gypsum)

 

 

 

 

 

 

 

Ceraform 126

0.27

3.70

³⁄₈"

(Johns-Manville)

 

 

 

 

 

 

 

19