
18
APPENDICES B & C
A B H V W:Natural
cord wood does not come with a specificaon sheet like
the building materials described in Appendix C. Manu-
factured products can be made to strict, repeatable tol-
erances. Natural wood, on the other hand, varies con-
siderably depending on where it grows, the climate, the
mineral composion of the soil, and more. Even sll,
considerable research and scienfic data is available on
different variees of wood. Added to the variaons in
the wood itself is the fact that fire wood is sold by vol-
ume; most typically by the cord. As discussed in Appen-
dix A, a cord measures 4' wide X 4' high X 8' long or 128
cubic feet. Beyond those facts it begins to get murky.
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. An apocryphal story aributed to a Vermont
wood burner avers that well stacked wood will have spac-
es 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 spac-
es are necessary to ensure proper drying. (FYI there is a
lively debate amongst experienced wood burners wheth-
er the bark side of the split wood should be up or down.)
Bark up or down the wood must be well seasoned (@
20% moisture) to be considered good firewood. Burning
green wood is inefficient and disappoinng; cung the
BTU values given below approximately in half.
Below is an abbreviated chart that gives some approxi-
mate weight and BTU values for various species of wood.
More comprehensive lists are available on line, but a
word or two of cauon. The stascs can vary widely
depending on the source or, more likely, on the point
they are trying to make. The numbers can be easily ma-
nipulated by changing assumpons. If a cord of wood is
assumed to have 30% air space then the esmated avail-
able BTU's will be considerably less. Likewise, assuming
that a pound of wood at 20% moisture contains say 8000
available BTU will net an appealing, inaccurate number.
In researching data we found the variaons considerable.
Use the numbers below as a point of comparison be-
tween species available to you, not as a score card. The
best advice is to find 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
and keep it dry, and enjoy clean efficient 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 Stan-
dard 1618; the Standard for Wall Protectors, Floor Protec-
tors, and Hearth Extensions, was adopted as a standard
in 2011. UL-1618 categorizes floor and wall protectors as
Type 1 or Type 2 protecon. A seemingly minor change
in UL 1618 calls for the actual thermal properes of floor
protecon to be specified in R-value in place of k-value.
In fact, that change makes it one step simpler to calculate
thermal protecon if it becomes necessary.
• Type 1: Ember Protecon = No thermal value
• Type 2: Thermal Protecon = R-value specified
Type 1 Ember Protecon is deemed to be a connuous
non-combusble material extending under the appliance
and to the front , sides, and back in the size specified. Any
thermal value a Type 1 protector may have is disregarded
under UL-1618. Type 2 Thermal Protecon provides ther-
mal resistance in addion to ember protecon. 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 floor protecon is to be built on-site, or
there is an exisng hearth already in place, then the ther-
mal resistance must be calculated. This is where having
thermal requirements expressed as an R-value vs. k-value
simplifies calculaons. R-values can be added together.
• R-value is a unit of measure of Thermal Resistance. It
is best known as the common measure of insulaon.
As with insulaon, the higher the number, the beer.
• k-value is a unit of measure of Thermal Conducvity.
k-value is the opposite (inverse) of R-value in that the
smaller the k-value the greater the resistance.
R-values for different materials can be added together to
determine the total R-value of composite layers of mate-
rials and k-values cannot. Converng k-value to R-value
is relave simple but it tends to add some 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 applicaons.) 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 materi-
al.) 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 protecon 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.
Today, with the R-value given, 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 protecon is sll not ad-
equate (R = 1.06). Add one more layer of 1/2" Durock and
the floor protecon is now acceptable (R = 1.32). (Brick,
le, etc. must be mortared in place, not loose laid.)
M R- -
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 specificaons and more detailed
explanaons of calculaons can be found online.