21
A
B C
A
B
̴
H
V
W
:
Natural cord
wood does not come with a speci
fi
ca on sheet like the
building materials described in Appendix C. Manufactured
products can be made to strict, repeatable tolerances. Natu-
ral wood, on the other hand, varies considerably depend-
ing on where it grows, the climate, the mineral composi on
of the soil, and more. Even s ll, considerable research and
scien
fi
c data is available on di
ff
erent varie es of wood.
Added to the varia ons in the wood itself is the fact that
fi
re wood is sold by volume; most typically by the cord. As
discussed in Appendix A, a cord measures 4' wide X 4' high
X 8' long or 128 cubic feet. Beyond those basic facts it gets
less scien
fi
c.
The amount of burnable wood that will be available in a given
cord can vary greatly on how it is cut, whether it is split, the
sizes of the whole or split logs, and the skill (or perhaps the
integrity) of the person stacking the wood. A bit of country
wisdom avers that well stacked wood will have spaces large
enough for a mouse to crawl through but not large enough
for the cat chasing him. Well stacked wood should be @
85% wood and 15% air space. The air spaces are necessary
to ensure proper drying. There is some debate among ex-
perienced wood burners whether the bark side of split logs
should be up (to shed water) or down (to promote evapora-
on). Bark up or down the wood must be well seasoned
(@ 20% moisture) to be considered good
fi
rewood. Burning
green wood is ine
ffi
cient and disappoin ng; cu ng the BTU
values given below approximately in half.
Below is an abbreviated chart that gives some approximate
weight and BTU values for various species of wood. More
comprehensive lists are available on line, but a word or two
of cau on. The sta s cs can vary widely depending on the
source or, more likely, on the point they are trying to make.
The numbers can be easily manipulated by changing as-
sump ons. If a cord of wood is assumed to have 30% air
space then the es mated available BTU's will be consider-
ably less. Likewise, assuming that a pound of wood at 20%
moisture contains say 8000 available BTU will net an appeal-
ing, inaccurate number. In researching data we found the
varia ons considerable.
Use the numbers below as a point of comparison between
species available to you, not as a score card. The best advice
is to
fi
nd an honest wood vendor and rely on them to deliver
a full cord of the best wood available. Buy your wood well
ahead of the burning season, dry it well and keep it dry, and
enjoy clean e
ffi
cient wood heat.
W
S
L .
C
BTU
C
American Elm
@ 3000 lbs.
@ 20,000,000
Apple
@ 3800 lbs.
@ 27,000,000
Cherry
@ 2925 lbs.
@ 20,500,000
Hemlock
@ 2700 lbs.
@ 18,000,000
Hickory
@ 4200 lbs.
@ 27,000,000
Red Oak
@ 3500 lbs.
@ 24,500,000
Shagbark Hickory
@ 4325 lbs.
@ 27,500,000
Sugar Maple
@ 3750 lbs.
@ 24,000,000
Tamarack
@ 3250 lbs.
@ 21,000,000
White Ash
@ 3500 lbs.
@ 24,000,000
White Birch
@ 3200 lbs.
@ 20,000,000
White Cedar
@ 1915 lbs.
@ 12,500,000
White Oak
@ 4200 lbs.
@ 29,000,000
White Pine
@ 2250 lbs.
@ 15,000,000
Willow
@ 2100 lbs.
@ 14,500,000
All values are approximate and for comparison only.
A
C
̴
F
W
P
:
UL Standard
1618; the Standard for Wall Protectors, Floor Protectors, and
Hearth Extensions, was adopted as a standard in 2011. UL-
1618 categorizes
fl
oor and wall protectors as Type 1 or Type
2 protec on. A seemingly minor change in UL 1618 calls for
the actual thermal proper es of
fl
oor protec on to be speci-
fi
ed in R-value in place of k-value. In fact, that change makes
it one step simpler to calculate thermal protec on if it be-
comes necessary.
• Type 1: Ember Protec on = No thermal value
• Type 2: Thermal Protec on = R-value speci
fi
ed
Type 1 Ember Protec on is deemed to be a con nuous non-
combus ble material extending under the appliance and to
the front, sides, and back in the size speci
fi
ed. Any ther-
mal value a Type 1 protector may have is disregarded un-
der UL-1618. Type 2 Thermal Protec on provides thermal
resistance in addi on to ember protec on. Thermal value
is determined by laboratory test. UL-1618 calls for thermal
requirements to be expressed as R-values rather than as k-
values which were used in the past.
If a hearth or
fl
oor protec on is to be built on-site, or there
is an exis ng hearth already in place, then the thermal re-
sistance must be calculated. This is where having thermal
requirements expressed as an R-value vs. k-value simpli
fi
es
calcula ons. R-values can be added together.
• R-value is a unit of measure of Thermal Resistance. It is
commonly known as the measure of insula on value. As
with insula on, the higher the number, the be er.
• k-value is a unit of measure of Thermal Conduc vity.
k-value is the opposite (inverse) of R-value in that the
smaller the k-value the greater the resistance.
Where R-values for di
ff
erent materials can be added togeth-
er to determine the total R-value of composite layers of ma-
terials, k-values cannot. Conver ng k-value to R-value is rel-
a vely simple but it frequently causes confusion. (It should
be noted that there are other units of measure, such as C-
value, Thermal Conductance, but they are not as commonly
used in hearth applica ons.) So, again, if only the k-value is
given, then it must be converted to R-value. (Keep in mind
that the
k-value is given per inch of material.)
To convert k-
value to R-value divide 1 by the k-value of the material and
divide by the thickness of the material.
C-values can also be
converted to R-values (1 ÷ C-value) and R-values can also be
converted to k-value (inches ÷ R) but by staying with R-values
materials can be added together.
Floor protec on requirements were once commonly ex-
pressed as "3/8" of asbestos millboard k = 0.84" or similar.
The conversion is: 1 ÷ 0.84 = 1.19 X 0.375" = 0.45 R-value.
With the R-values known, consider this example:
A wood heater requires R = 1.19. If 4" common brick is the
material of choice; their R-value is 0.80 (0.20 X 4"). Add 1/2"
of Durock (R = 0.26) and the protec on is s ll not adequate
(R = 1.06). Add one more layer of 1/2" Durock and the
fl
oor
protec on is now acceptable (R = 1.32). (Brick, le, etc. must
be mortared in place, not loose laid.)
M
R-
k-
Common Brick
R = 0.20 per inch
k = 5.00 per inch
Durock
R = 0.52 per inch
k = 1.92 per inch
Hardibacker
R = 0.51 per inch
k = 1.95 per inch
Micore 160
R = 2.86 per inch
k = 0.86 per inch
Marble
R = @0.09 per "
k = @11 per inch
More complete lists of material speci
fi
ca ons and more detailed
explana ons of calcula ons can be found online.
