|
Architects, contractors and
builders use various "factors" to express the insulation
value of a material or a composite structure: K, C, U, and R. Material
suppliers often speak of products having a particular K
factor.
The purpose of this document is to explain what these
factors mean, how they differ from each other, how they
relate to each other and to clear up the
R value or R
factor Myth.
K,
factor
K Factor is a rate at which heat flows
through a material or insulation . Values for insulation are normally based
on one-inch thickness of one homogeneous material, and are
expressed in BTU/ft2/F/hr./inch. (note: In the United
States, the U-factor is usually expressed in Btu/h ft² F.
The metric equivalent unit is W/m² K.)
The lower the K factor, the better the insulation value of
the material. Some examples are:
|
Vermiculite |
K= .50 - .60 |
| Fiberglass: |
K = .22 - .30 |
| Urethane Rigid Foam |
K = .11 - .16 |
These values are shown as a range because
the K Factor does change with age. For example, urethane
foam insulation exhibits a K of .11 or .12 when new. In two to six
months, the K increases to .15 or .16, then remains stable.
C,
factor
C Factor is also a rate of heat through a
material, but could be for any given thickness, not just at
1 inch. C Factor at 1 inch would be equal to the K factor. C
factor of the same material at three inches thickness is 1/3
of the K factor; at two inches, it is 1/2 the K factor.
Wow!.
Are you getting all this C factor and K factor stuff? Don't
worry, by the time your done reading this page you will know
all about "R-value", R factor, C factor, K factor and U
factor. Speaking of U factor, lets move on and discuss
U factor now.
U,
factor
U factor is the overall coefficient of heat
transfer (conductivity) for all the elements of
construction, as well as the environmental factors.
Said another way, U factor is
the coefficient of transmission, i.e., the transmission of
heat through the materials, which compose the building's
"envelope," or outer shell. An
example would be the U factor of a composite structure such
as qypsum wallboard, fiberglass core and exterior wood
sheathing. The U factor should not be used with a single
material, only with combinations.
A U factor can also be determined by adding the C
factors of the
various individual materials making up a composite
structure. U=C1+C2+C3, etc. If the materials are not in
close contact, the C factor of an air space must be included
in the calculation. Units are expressed as BTU/ft2/f/hr.
The smaller the U factor, the better the insulation value of
the composite structure.
U factor has an inverse
relationship to R factor. For example, a building with
material with an R factor of R-11 converts to an U factor of
0.09 ( 1 divided by 11 ).
Lets use a window as an example
in our U factor discussion:
The U-factor measures how well
a product prevents heat from escaping. The rate of heat loss
is indicated in terms of the U-factor of a window assembly.
U-factor ratings of windows generally fall between 0.20 and 1.20.
The insulating value (factor) is indicated by the R-factor, which is
the inverse of the U-factor. The lower the U-factor, the
greater a window's resistance to heat flow and the better
its insulating value (R value or R factor). The U-factor is
included in the energy performance rating (label) offered by
the National Fenestration Rating Council (NFRC).
Window
U-factor. The rate of heat loss is indicated in terms of the
U-factor of a window assembly. Remember, the lower the
U-value, the greater a window's resistance, insulation
resistance as
well, to heat flow and the better its insulating value. Also
this
would equate to a higher R factor or value, which is the
inverse of the U factor remember.
Northern Climate: Select windows with a U-factor of
0.35 or less. If air conditioning loads are minimal, windows
with U-factors as high as 0.40 are also energy-efficient if
the Solar Heat Gain Coefficient is 0.50 or higher. Some
double-glazed low-e products have U-factors below 0.30. Some
three-layer products have U-factors as low as 0.15.
North/Central Climate: Select windows with a U-factor
of 0.40 or less. The larger your heating bill, the more
important a low U-factor becomes.
South/Central Climate: Select windows with a U-factor
of 0.40 or less. The larger your heating bill, the more
important a low U-factor becomes.
Southern Climate: A low U-factor is useful during
cold days when heating is needed. A low U-factor is also
helpful during hot days when it is important to keep the
heat out, but it is less important than SHGC in warm
climates. Select windows with a U-factor lower than 0.75 and
preferably lower than 0.60.
OK, enough
about windows and their U factor and R factor.
We are almost done. Just one last factor to learn; the R
factor. So let us move on to the the R
factor or R value calculation or equation.
R,
factor
R factor
is the most discussed and misunderstood factor in
insulation.
R factor is a measure of the resistance to
heat flow. R factor can be determined for a single
insulation at a
specific thickness. As the thickness of insulation
increases, the resistance to heat flow increases if Ceterius Paribus.
Ceterius Paribus;
Cēterīs paribus is a Latin phrase, literally translated
as "with other things [being] the same," and usually
rendered in English as "all other things being equal."
R
factor can be determined in TWO ways:
1.) R factor = 1 divided by the C factor of the material.
2.) R factor = the thickness divided by the K factor.
Example:
With rigid urethane foam at 3-inch thickness R factor = 3 / .15 =
20 OR could be stated to have an R value of 20 / R20 Value
At two inches thickness R factor = 2 / .15 = 13
OK, that is the BASIC
calculation or equation of R factor. However, yes
now for the however, in the
real world environment there are
variables (U factor) that change how that insulation will perform
(R factor or R value). Read the
R
factor Myth when you're done reading this for the
detailed version of what is below.
By now you understand why
we said," ... if Ceterius Paribus". Because
of the U factor, K factor and C factor not all
insulations perform the same in the real word.
(hint: think wind, moisture, and temperature.)
R-value or R factor
numbers are "funny" numbers or
Myth
Numbers.
Example:
The test used to produce
the R value is an ASTM test.
This ASTM test was designed by a committee to give us
measurement values that hopefully would be meaningful. A
major part of the problem lies in the out dated design of the
test. The test favors the fiber insulations --
fiberglass, rock wool, and cellulose fiber. Very little
input went into the test for the advanced solid insulations
of today, such
as foam glass, cork, expanded polystyrene or urethane
foam. Which have much higher K factors and U
factors.
The test does not account for air movement (wind) or any
amount of moisture (water vapor); the U factors, K
factors or C factors of state-of-the-art spray foam
insulations. In other words, the
test used to create the R-value is a test in non-real-world conditions. For instance, fiberglass is
generally assigned an R-value of approximately 3.5. It
will only achieve that R-value if tested in an absolute
zero wind and zero moisture environment (U factor).
Let alone temperature changes (more on temperature in
R
factor Myth). Zero wind
and zero moisture are not real-world. Our houses
leak air, all our buildings leak air, and they often
leak water. Water vapor from the atmosphere, showers,
cooking, breathing, etc. constantly moves back and forth
through the walls and ceilings. If an attic is not
properly ventilated, the water vapor from inside a house
will very quickly semi-saturate the insulation above the
ceiling promoting mold . Even small amounts of moisture will cause a
dramatic drop in fiber insulation's R value -- as much
as 50 percent or more (because of the impact of the U
factor. Remember U factor is conductivity).
 
From this table, it can be seen that rigid
urethane foam is 60% better than fiberglass and 350% better
than Vermiculite when all are the same thickness.
"R-value", R factors, U factors, K factors
and C factors, can be obtained from material
suppliers, lumber yards, and Engineering handbooks.
Although if your having difficulties locating a materials K factor, C factor,
U factor, or R factor (R Value),
contact us and well will gladly provide you with any
documentation we have on hand. Please note, this is not an
invitation for us to do research for a materials K factors, C factors,
U factors, or R factors (R Value). It is provided as a quick and abundant source of information that we
have on hand about a materials factors. Again, of
information we have on hand, if we have it we will gladly
share it, if not, then you will need to do a little research.
The
U.S. Federal Trade
Commission developed an insulation rating mechanism to help end
the false and misleading claims about insulation that once
abounded. Called
R-value, this rating is based on a laboratory
measurement, the R-factor, which reflects a material’s ability
to stop or slow heat transfer.* When you’re shopping for
insulation or talking to your builder, understanding "R-value", R
factor, U factor, K factor and C factor, can
help you make intelligent choices about insulation.
If you don't see your solution listed,
contact an All Tech Industries
engineer today (989-826-9999) for a deeper discussion
about your insulation requirements.
|
