Volume 37, Issue 8, August 2002


Specialty Windows
        The Shape of Things to Come
by Ric Jackson

Today, nearly every new single-family home has at least one feature window that is a non-rectangular/specialty one. This does not include the increase in entry door system panels, which are not tracked or reported. According to the 1999 U.S. and Canadian Window and Door Market Research Report prepared by Ducker Research Co., specialty shapes and sizes have become popular, with more than three million specialty units on the market.

What percent of the IG market is now flexible?
Along with specialty windows comes a host of market opportunities for products needed to produce “specials,” such as flexible spacers.

According to the 1999 Ducker report, the category of flexible warm-edge spacers has already captured 28 percent of the total residential insulating glass (IG) edge seal market. Discriminating consumers like the curves and unusual angles made possible with today’s framing and IG materials. This alone should be enough reason to consider incorporating flexible IG spacers into your production process, but, more importantly, the push for standardized energy rating performance advances across North America is sparking this trend. 

The recent endorsement of the National Fenestration Rating Council (NFRC) window labeling program by the National Association of Home Builders continues the trend of growing interest in standardized reporting of the thermal performance of windows. According to the NFRC, there are 85,000 products on the market today that employ the NFRC label. This, combined with publicity from the ENERGY STAR® program, adds a consumer-friendly element to a complicated valuation process. 

The rating values reported for a given window design have to be based on the actual construction. So, if a rectangular unit has a rigid spacer that is less energy efficient than a round-top unit built with a flexible spacer of better thermal performance, the lower-performance rigid spacer will affect the rating negatively. With a few exceptions (such as thermally broken and stainless steel), rigid spacers are less energy-efficient than their newer, flexible counterparts. This is due in part to the use of less conductive materials.

Most rigid “tube” or “box” spacers can be bent at some cost and difficulty. In today’s competitive market, however, adding cost or delaying delivery time is a significant problem. If you add to this the continuing education of consumers by the industry and their ever-increasing sensitivity to anything that detracts from aesthetics, it will only be a matter of time before consumers question the use of different types of spacers in windows of mixed construction.

What effect can the spacer have on the energy rating of the window?
Modeling of a typical double hung (vertical slider) window with high performance low-E glass and Argon-filled IG units can improve up to 4 percent in condensation index, 6 percent in U-value and 17 percent in edge of glass temperature from the poorest performing rigid warm edge1 to the best2 performing flexible warm edge3.

Is there a penalty for using flexible spacers?
In the early stages of the technology there might have been some limitations in terms of productivity for the largest producers to use flexible spacers, but today, there are numerous equipment platforms with varying degrees of automation and unique application tools that take the variability out of working with flexible spacers. Most companies switching to flexible spacer systems see cost decreases for a variety of reasons as well as an opportunity to market the thermal performance benefits. 

1 “U” Channel tin plated steel spacer.

2 Hollow, Flexible, non-metallic spacer.

3 Simulations were performed by Enermodal Engineering Limited using Window 5 beta 2 and THERM 5 alpha. The windows used in these simulations were not real windows but were generic designs that represent real windows.


jackson Ric Jackson is director of marketing for TruSeal Technologies Inc., based in Beachwood, Ohio.


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