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Why R is Not Simply the Inverse of U
by Michael O’Brien
Quick question: which is better—something rated 0.2 or 5?
When the Department of Energy introduced its “R-5” window program, some
folks in the window industry might have done a double-take. After all,
window thermal performance rating systems have always used the term “U-value”
or “U-factor” to define window thermal performance. The latter, in fact,
is clearly emblazoned on the National Fenestration Rating Council (NFRC)
thermal performance label. So, why the sudden reference to “R-value?”
Many consumers have heard of R-values—a measure of a product’s resistance
to heat flow—and easily grasp that materials with higher R-values, such
as R-19 vs. R-11 wall insulation, are more energy-efficient. That is,
the better the product is at stopping the flow of heat to the outside
during winter or to the inside during summer, the easier (and cheaper)
it is for HVAC systems to maintain the desired indoor temperature for
a longer time.
Usually, an R-value is given for a certain thickness of a specific homogenous
material such as fiberglass insulation. The R-value of a structure that
is made of layers of different materials (exterior sheathing, insulation,
interior dry wall, etc.) can be estimated simply by adding the R-values
of the individual layers.
While R-value measures resistance to heat transfer, U-value measures the
rate of heat transfer. The lower its U-value, the better the product’s
ability to resist heat conduction.
In simple terms, U-value is the mathematical reciprocal of R-value; that
is, U = 1/R and R = 1/U. For instance, a material with an R-Value of 5
has a U-value of 0.2 (1 divided by 5). As the R-value goes up, the U-value
goes down, and vice-versa. However, it actually is more complicated than
that.
Known officially as thermal transmittance, U-value is more of an engineering
term used to designate the thermal performance of a system as opposed
to that of a homogeneous material. U-value thus has been used traditionally
to express the thermal efficiency of windows, which, unlike wall insulation,
are complex assemblies of components with a variety of sometimes conflicting
missions.
"As the
R-value goes up, the U-value goes down, and vice-versa. However,
it actually is more complicated than that."
Wall insulation is singularly intended to limit heat transfer,
making it fairly straightforward to assess insulation strictly on its
ability to achieve that objective. However, windows may need to permit
ventilation while limiting unwanted air infiltration and admit daylight
while optimizing the effects of solar heat gain. Since R-value currently
is applied to homogenous materials with a singular purpose and is understood
in the marketplace based on such products, it may not be equitably applicable
to multi-purpose products with varying construction, such as windows.
U-value accounts for how energy enters and leaves the material; it considers
both conduction and radiation. R-value accounts only for the resistance
to heat flow by conduction.
Per NFRC 100-2010, Procedures for Determining Fenestration Product U-Factors,
the overall U-value of a window product is a prorated summation of U-values
of the center of glass, edge of glass and frame areas. This takes into
account such details as insulating glass edge spacers, certain hardware
and frame cross-section.
Commercially, R-value is used to define the energy efficiency of many
building materials because it is intuitively easier for consumers to understand
that R-19 insulation is better than R-11, rather than trying to explain
how U=0.05 insulation is better than U=0.09. However, in the case of windows,
using U-values avoids comparing a window’s multi-faceted overall performance
to the purely insulating value of a wall.
So, while it is unusual to see window energy efficiency expressed in terms
of R-value, it does relate well to the average consumer’s understanding
of insulating capability. However, it also could lead to confusion since
R-value doesn’t take into account all of the facets of a fenestration
product the way U-value does. (For more on U- versus R-values, see July-August
DWM, page 21.)
Ken Brenden serves as technical standards manager
for the American Architectural Manufacturers Association in Schaumburg,
Ill. He may be reached at kbrenden@aamanet.org. His opinions are solely
his own and do not necessarily reflect those of this magazine.
DWM
© Copyright 2010 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.
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