The Next Frontier
Thermal Optimization of Residential Windows
Editor's Note: The following article is adapted from a white paper
by Mike McHugh, president of Integrated Automation Systems in Solon, Ohio,
which was presented during the recent FeneTech Annual User Conference
The insulating glass (IG) business has gone through continuous
transformation in the last 30 years. With all the improvements that have
been introduced to our industry, it is difficult not to be impressed.
Although each was often labeled “incremental” and not worth the investment,
the accumulation of these options have resulted in a much higher performing
and better quality window than produced in the past. It was never a question
of whether a fabricator was going to adopt these changes—only when! Competition
and the free market have relentlessly driven our industry to improve.
Measuring performance has been a great concern in the past.
Claims were made by individual companies, and it was largely left to consumers
to decide who was right and who was misleading. Terms such as emissivity,
spectrally selective, warm edge, krypton, enhanced and center-of-glass
values all sounded good in a sales pitch, but what did they mean? Energy
Star has provided a platform to answer this question. It has created a
level playing field on which our industry must compete.
At last, we can all strategize exactly where on the cost
value curve we would like to reside in the design and construction of
high-performing fenestration products. While there have been way too many
changes to explore in this document, the accumulation of these changes
now presents us with a new challenge: How do we best combine all these
improvements to produce the highest performing window that is both durable
and cost-effective? Among many options to consider are:
• Hard coat or soft coat;
• Single, double or triple silver coating;
• 4th-surface coatings in two-pane units;
• 6th-surface coatings in three-pane units;
• Double-pane vs. triple-pane;
• Symmetric vs. asymmetric;
• Edge effect of spacer;
• Argon, or krypton, or air;
• Argon, and krypton, and air; and
• Overall IG unit thickness.
Scope and Purpose of Analysis
The purpose of this analysis is not only to provide specific answers to
the cost/value question, but also to demonstrate a methodology to evaluate
the cost of different IG process options and compare it with the resulting
U-value for a generic vinyl window. Although your window is not generic,
and—and it may not even be vinyl—using this model for all comparisons
enables you to gain a sense for alternative ways to improve your window
using your IG package. Using this methodology provides a better understanding
of both the cost and ability to affect performance. It is exciting to
be conducting an analysis that does not discuss “center of glass” or even
the U-value of an entire unit, but only its contribution as a component
of a window.
Segments and Base Unit for Cost Comparison
To help make sense of this analysis, the market was broken into three
segments based on window glazing pockets:
• 3/4-inch units;
• 7/8-inch units; and
• 1-inch units. The base unit is a 3/4 -inch two-pane with:
• 2.5-mm clear panes (for purposes of the test Quanex used Clr-28.CIG
• Stainless steel spacer;
• 0.563 air space;
• Air-filled; and
• 2.5-mm double-stack low-E (for this purpose we used PPG Solarban 60).
The 3/4 -inch unit above is used as the base. The cost
to upgrade from this base to the different unit designs considered was
then taken into account. The same incremental costs were used consistently
for all designs. Keep in mind that if you regard the process as a credible
way to make evaluations, it would be relatively easy to substitute your
particular numbers for cost considerations.
Cost for Upgrades to Achieve Targeted U-Values
The generic window in question incorporates 10 square feet of glazing.
The assigned upgrade option costs below are per window:
• Additional double silver coating: $3.70;
• 4th-surface coating: $6;
• Spacer upgrade for each 1/8 inch: $0.80; and
• Estimated cost to go to a triple IG: $10.
Cost Performance Measurements
We would like to thank Quanex Corp. for its assistance in modeling the
performance of the various units in this presentation. The intention is
to show not only what U-values can be achieved, but to display (by segment)
the incremental cost of achieving each particular U-value. It was an intention
to use this process to ultimately depict the optimized (cost vs. performance)
U-value opportunities in the three different window packages mentioned.
Conclusion: R-5 Windows are Cost-Effective
• Whatever glazing pocket size you make, thermal optimization will play
a significant role in your product development.
• There are lots of options using the noted combinations, and no wrong
• If there is a wrong answer, it is that producing an R-5 window is
not cost-effective. With manufacturing cost premium in the range of
$2 per square foot of glazing, the industry can provide solutions to the
rising cost of energy, as well as CO2 reductions.
• The incremental changes have not stopped happening. The industry is
continuing to develop better performing products for our customers and
the outlook is positive!
• It is clear Energy Star has leveled the playing field. Window companies
that win in this new competition will be incorporating thermal optimization
as tools for developing future products.
© Copyright 2012 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.