Lennox Hearth CR-3835R Hearth Extension Dimensions, 20 508 mm, 38 965 mm, 8 203 mm, 12 305 mm

Hearth Extension Dimensions
A	20" (508 mm)
B	38" (965 mm)	Metal Safety Strip
C	8" (203 mm)			8"
		12"
		(305mm) E		(203mm)
D	20" (508 mm)	Min.	D	C
E	12" (305 mm)	20" B	A 20"
		(508mm)	(508mm)
Figure 43		Outside Corner

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?

kM = k value per inch of alternate material rM = r value per inch of alternate material

TM = minimum thickness required for alternate material TS = standard thickness of the alternate material

kL = k value per inch of listed material rL = r value per inch of listed material

TL = 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 (kM) x		thickness
alternate		k-value (per inch)		of listed
material (TM)		of listed material (kM)		material (TL)
TM (inches)	=	kM	x	TL
		*.84
TM (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
TM (inches)	=	rL	x	TL
		rM
TM (inches)	=	*1.19	x	1"
		*2.86
.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”.

Listed Material

	k (per inch)	r (per inch)	Listed Min.Thickness
Listed Material	.84	1.19	1"
	KL	rL	TL
** Approved Alternate Materials for Floor/Hearth Protection
AlternativeMaterials	Thermal Values		MinimumThickness
			(rounded to nearest 1/8 inch)
	k (per inch)	r (per inch)	Min. Thickness
	KM	rM	TM
Kaowool M Board	.47	2.13	.56" (1/2")
Micore 160™	.35	2.86	.42" (3/8")
U.S. Gypsum
Micore 300™	.46	2.17	.55" (1/2")
U.S. Gypsum
Durock™ Cement Board	1.92	.52	2.29" (2-1/4")
U.S. Gypsum
Hardibacker™	1.95	.513	2.32" (2-3/8")
Hardibacker 500™	2.30	.435	2.74" (2-3/4")
Cultered Stone Hearth-	2.82	.355	3.36" (3-3/8")
stone™
Wonderboard	3.23	.31	3.85" (3-7/8")
Super Firetemp M	.61	1.64	.73" (3/4")
Johns-Manville
Super Firetemp L	.54	1.85	.64" (5/8")
Johns-Manville
Face brick	9.00	.111	10.71" (10-3/4")
Common brick	5.00	.20	5.95" (6")
Cement mortar	5.00	.20	5.95" (6")
Ceramic tile	12.5	.08	14.88" (14-7/8")
Marble	11	.09	13.10" (13-1/8")
Table 1

**If the hearth extension material(s) that is intended to be used is NOT listed on Table 1, the material can still be used if the material(s) is noncombustible. However, the manufacturer of the material must provide either the listed k- value per inch or r-value per inch with listed thickness so that the minimum thickness required for the hearth can be calculated (per instructions on this Page and/or as specified in the NFI Certification manuals).

Note: Also see NFI (National Fireplace Institute) Certification Manuals showing other acceptable calculation methods and acceptable alternate materials which can be used.

NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.

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