Page 38
433-1390G
September 1, 2008
R
7100FP EPA Woodburning Fireplace
8
Finishing
A. Hearth Extension
A hearth extension must be installed with all fireplaces to
protect the combustible floor in front of the fireplace from
both radiant heat and sparks. See Figures 38.1 below, and
Figures 39.1 and 39.2 on page 39.
2 in. (51 mm)
required
High temperature
(300°F min.) continuous,
non-combustible sealant
Tile, stone or other
non-combustible material
Floor constructed of wood or
other combustible material
HX3 or HX4
Hearth Ext
or equivalent
insulation
(see Table below)
Fireplace
Protective
Metal Hearth
Strip
Figure 38.1 Hearth Extension Construction
Table 38.2
Hearth Extension Insulation Alternatives,
Minimum R Value = 2.06
Material
k per inch
thick
R per inch
thick
Minimum
thickness
required
Hearth & Home HX3, HX4
0.49
2.06
1 in.
USG Micore 300™
0.49
2.18
1 in.
USG Durock™ Cement Board
1.92
0.52
2-1/4 in.
Cement Mortor
5.0
0.20
5-7/8 in.
Common Brick
5.0
0.20
5-7/8 in.
Ceramic Tile
12.50
0.08
14-5/8 in.
Armstrong™ Privacy
Guard Plus
0.46
2.18
1/2 in.
Marble
14.3-20.0
0.07-0.05 16-5/8 in. - 23-3/8 in.
B. Calculating Alternate Floor Protection
Material
Thermal Conductivity: k value
The k value indicates the amount of heat (in BTU’s) that will flow
in 1 hour through 1 square foot of a uniform material 1 inch thick
for each degree (F) of temperature difference from one side of
the material to the other. The LOWER the k factor means less
heat is being conducted through the non-combustible material
to the combustible material beneath it. The k value of a mate-
rial must be equal or smaller then the required k value to be
acceptable.
(BTU)
(inch)
(foot
2
(hour) (
o
F)
Thermal Resistance: R value
The R value is a measure of a material’s resistance to
heat transfer. R value is convenient when more than one
material is used since you can add the R values together,
whereas you can not do this for k value. The HIGHER the R
factor means less heat is being conducted through the non-
combustible material to the combustible material beneath it.
The R value of a material must be equal or larger then the
required R value to be acceptable.
Converting k to R:
Divide 1 by k and multiply the results times the thickness in inch-
es of the material.
R = 1/k x inches of thickness
Converting R to k:
Divide the inches of thickness by R.
k = inches of thickness/R
Calculations:
Example: Floor protection requires k value of 0.84 and 3/4 inch
thick.
Alternative material has a k value of 0.6 and is 3/4 inch thick.
Divide 0.6 by .75 = k value of 0.80. This k value is smaller than
0.84 and therefore is acceptable.
Fire Risk
• Maintain
clearances.
• Use only non-combustible material below
standoffs, material such as cement board
is acceptable.
WARNING
• Framing or finishing material used on the front
of, or in front of, the appliance closer than the
minimums listed, must be constructed entirely of
noncombustible materials (i.e., steel studs, concrete
board, etc.).
