Industry Discusses U-Factor Versus R-Value
for Window Performance
June 21, 2010
Historically, the term U-factor, which measures the rate of heat
transfer of a material, has been used to explain window performance.
However questions have risen as to whether the term R-value, which
measures the thermal resistance of a material, could be used as
Dr. Brandon Tinianov, P.E., LEED AP, chief technology officer with
Serious Windows, has written a paper titled "The Use of R-value
Versus U-value to Describe Window Performance," and states
"if the use of U-value is established, why would one have a
desire to use R-value as an equivalent alternative?"
Tinianov explains that first, "U-values [U-factor] are small,
usually less than one, with diminishingly smaller values as performance
improves. In contrast, R-values are presented in a number range
of highest comfort for a consumer-between 1 and 10 (possibly 20)
Second, the inverse relation of U-value and performance is
counter intuitive. As u-value diminishes, performance increases."
He also states, "There is good public reason and good technical
precedence for the interchangeable use of R-value and u-factor to
describe windows. With only slight modification to terminology associated
with R-value, the public will be empowered to make smarter, more
intuitive energy efficiency decisions. Additionally, product manufacturers
and building designers will have another useful method of describing
their materials in a fair and transparent marketplace." (CLICK
HERE to read the full paper).
Recently, the National Fenestration Rating Council (NFRC) sent
out a bulletin explaining why it chooses to use U-factors for windows.
In the bulletin, Jim Benney, NFRC chief executive officer, says
"From a technical perspective, there are numerous philosophies
about whether R-value applies only to homogeneous materials and
should be measured in terms of surface to surface heat transfer
- i.e., making it the true inverse of conductance - by a guarded
hot plate (ASTM C177). Or, should it be used for composite materials
and measured in a calibrated hot box in accordance with ASTM C236?
Or, should it be measured by means of a heat flow meter (ASTM C
Benney continues, "U-factor is not a material property value.
It is the result of a calculation that combines the conductance
values of the numerous materials in a fenestration product. This
includes glazing materials, gas fills, spacer materials, framing
materials, weather strips, sealants, etc. In addition, it includes
the convection and radiation elements that occur within and adjacent
to the fenestration product surfaces that dramatically influence
its energy rating."
Benney adds, "It is critically important that product performance
is communicated consistently to all interested parties. U-factor
is the recognized term for relating the thermal transmittance of
windows, doors, skylights, curtainwalls and fenestration attachment
products. NFRC will continue to recognize U-factor-and U-factor
only-for fenestration products."
Some window and energy experts in the industry also have thoughts
on the matter.
"U-factor takes into account not just conduction but also
airflow, absorption and radiation (emissivity). Unlike most building
materials that use an R-value rating and are made up of a single
material component (such as insulation, roofing materials, etc.),
windows are made up of many components that create the window assembly
and the U-factor more accurately measures the heat transfer of this
assembly of components," says Kerry Haglund, a senior research
fellow with the Center for Sustainable Research, University of Minnesota.
"Because windows allow for the visible and physical connection
between interior and exterior, airflow, absorption and radiation
are important not only for energy-efficient reasons, but also for
human factor issues such as thermal comfort and views to the outside."
Tom Culp, with Birch Point Consulting LLC, adds, "While R-value
does have greater meaning to consumers, there are also technical
issues when applied to windows which have the potential to also
mislead consumers if oversimplified information is given."
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