Volume 13, Issue 7 - September 2012

feature

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 in Cleveland.

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 measuring .110);
• 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 answers.
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.


DWM

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