With a new bag of tricks, double-pane says “not so fast”

By Drew Vass

It’s been a long-lasting relationship—the love between door and window manufacturers and double-pane, insulating glass (IG). As the most straightforward way to carve out energy efficiency among windows, the product has been a go-to standard for around half a century. But with developments and market penetration for gas fills and low-emissivity (low-E) coatings reaching their saturation points, is double-pane’s era coming to an end?

With the costs for certain materials decreasing and the newest technologies still underutilized, experts say chances are double-pane IG has years left. And depending on how you classify the latest IG products, some experts suggest it could be more like decades.

The Fight for Longevity

Some sources trace the concept for double-pane IG back to the 1860s, when they say an inventor filed a claim with the U.S. Patent and Trademark Office for a product consisting of two glass panes separated by rope (a primitive spacer system) and bound with tar (an adhesive and sealant). As late as the early 1920s through the 1950s, however, such a concept was precluded by the promotion of storm windows as an add-on for single-pane glass. In the mid- to late-1950s, double-pane glass finally had its day among advertisements (and public awareness), when PPG began promoting its “Twindow,” declaring it the “world’s first insulating glass.”

No sooner did double-pane IG reach mainstream adoption, however, than manufacturers began tweaking its properties with new technologies to give it an edge over other impending concepts—including the invention of low-E coatings in the 1970s, which some researchers have described as “one of the most important innovations of the flat glass industry in the past century.” Amid an energy crisis leading into the 1980s, low-E coatings and argon gas fill helped to stave off the necessity for triple-pane windows. Minus those developments, by that point, you could argue that double-pane IG was, in principle at least, at the end of its lifetime, argues Stephen Selkowitz, an expert in window-related technologies and affiliate for the Building Technology and Urban Systems Division at Lawrence Berkeley National Laboratory (LBNL). “But by adding low-E and argon, we extended [it] another 20 to 30 years,” he says, increasing thermal performance by a factor of about 50%.

And so the story goes for double-pane IG, every so many years.

Krypton gas, warm-edge spacer systems, surface-four low-E coatings—amid the pressures of competition, double-pane has always found a way to bounce back, widening its sweet spot over competitors like triple-pane glass, IG with suspended films and now vacuum insulating glass (VIG).


For this reason, even as triple-pane has taken hold in places such as Europe and Canada, here in the U.S., double-pane remains the industry standard among residential windows—partly due to milder climates, experts suggest. Cost is also an issue, as manufacturers weigh the option for retooling their systems for thicker, heavier glass.

“Sadly, it seems most windows are sold on the basis of cost, so the margins would be even tighter with triple-glazed units over double-glazed,” explains Margaret Webb, executive director for the Insulating Glass Manufacturers Alliance (IGMA).

While IG manufacturers in Europe are set up to produce primarily triple-pane glass, you can bet that U.S.- based companies will stick with what works, says Kyle Sword, business development manager for Pilkington. After eking out as much performance and as many years as possible from double-pane IG, Spencer Culhane, senior building envelope specialist for Schuco, says that he suspects most U.S.-based manufacturers will “leap over” triple pane, instead advancing to other options that don’t require as many structural changes to their products.

Other options offering the same or better advancements in thermal performance as triple-pane have come along without the added weight and thickness but remain niche so far. Some of those products also include two—not three—lites of glass, begging the question: Are they completely new technologies, or just the latest iteration of double-pane IG?

VIG, for instance, includes two lites of glass separated by a thin space. While standard, double-pane IG holds two lites of glass together using spacer systems, filling the resulting space with inert air or specialized gases, VIG relies on spacers to hold two lites apart, while vacuuming out all of the air between, creating a void.

At as little as 6-mm thick, the product is a far cry from triple-pane IG in weight and thickness. So far as VIG’s performance is concerned, in its original iteration, the product was, “not greater than some high-performance double-pane insulating glazing systems,” suggested LBNL researchers. With the latest VIG products reaching as much as R-14, that’s changed, but at a rate of what Selkowitz says is an additional $20-$25 per square foot of window (compared to double-pane), even those performance gains have yet to be justified for typical windows. For now, even manufacturers of VIG readily admit that the product is in its introductory phases and best suited for niche markets. In the meantime, after decades in the making, another form of IG that Selkowitz refers to as “Thin Triple” is prepping to enter the marketplace. (In full disclosure: Selkowitz is one of many scientists who have worked to develop and advance “Thin Triple” over the years, and who are working with door and window manufacturers to help bring the concept to market.)

Options Are Thin

LBNL’s Thin Triple includes three lites of glass, with an inner lite that’s made of the same thin glass used in flat-panel televisions. The product also utilizes krypton gas and low-E coatings on two surfaces to achieve R-8 performance (roughly double the performance of double-pane IG) in an IG unit that’s roughly the same weight and thickness as standard, double-pane glass.

At the same time, similar to VIG, there’s some debate to be had over whether Thin Triple is a new form of triple-pane, or a new iteration of double-pane IG. That argument hinges on how the inner lite of glass fails to form two distinct air spaces, the way that triple-pane IG does.

The construction of Thin Triple glass takes on several possible forms, Selkowitz says, not always including two spacer systems, the way that standard triple-pane IG does, to form two sealed air spaces. Instead, the technology incorporates a passageway (often a tiny hole), conjoining the two air spaces surrounding its inner lite of glass, allowing for equalization between the two spaces, he says. For this reason, one could argue that Thin Triple is in many ways just a new version of double-pane glass, with a third inner lite of glass, “dropped in,”  he admits.

For around the past 30 years, that same concept has been utilized through a technology referred to as IG with “suspended film” (some of which has been branded as Heat Mirror). IG with suspended film includes one or more layers of thin film, applied much in the same way that Thin Triple utilizes thin glass. And while the technology has in some cases produced higher performance ratings than standard, double-pane IG, for decades it’s been unable to take its place as a mainstream standard.

Unlike traditional triple-pane glass, independent glass experts say that both Thin Triple and IG with suspended films aren’t dogged by issues of weight and proportion the way that triple-pane glass has been. So far as Thin Triple is concerned, the product also isn’t dogged by price, as VIG is, Selkowitz says. Thin Triple adds $2-$3 per square foot and is capable of fitting within the same extrusions made for double-pane glass, he says.

The Race is On

In the end, the equation for replacing double-pane IG as a standard among windows is fairly simple: a product need only to offer better performance, at a cost that justifies its gains. But just as the prevalence for double-pane IG with .30 or better U-factor and solar heat gain coefficients was ushered in via tax credit requirements, forcing the industry to reach beyond its comfort zone is something that’s only likely to happen via more stringent codes, experts suggest. Sword suggests that could start in Canada, as the country hunkers down on performance requirements. Other driving factors that are lacking in the U.S. include high energy costs, which helped to make triple-pane a standard in Europe, industry experts say.

At the same time, based on most U.S.-based climates, even with some existing products spanning as high as R-14 in performance, there is a break-even range for total building performance, Selkowitz and Mark Meshulam, of Mark Meshulam LLC, a building envelope consultation, testing and repair firm that’s based in Northbrook, Ill., both suggest. From a perspective of practicality, Selkowitz says, “An R-8 glazed window is at the point where it’s kind of energy-neutral. In a cold climate, it saves as much energy in the winter as it loses, so it’s almost what we call a net-zero effect.”

And while Culhane says that it would be relatively inexpensive for his company to deploy the same high-performance window frames that it uses in Germany here in the U.S. (with substantially improved thermal breaks, he says), such a move hardly makes sense when existing U.S.-based products meet and even exceed current standards.

More Lite to Live

In the meantime, even excluding Thin Triple and VIG as their own technologies, manufacturers and experts agree that the latest IG technologies are capable of extending the life of double-pane for years ahead. Kevin Anez, director of product management for Milgard, says that the U.S. market has yet to fully deploy double-pane glass with surface-four low-E coatings. At the same time, Selkowitz says the costs for krypton gas, which traditionally has been reserved as a premium upgrade, has decreased from $2-$3 to around .50 cents per square foot over the past five years. Those features alone are likely to become standard for double-pane IG, helping it to meet impending code requirements, before manufacturers will move on to more expensive options.

Webb suggests that double-pane has at least another five years in it. Bill Lingnell, an independent expert with Lingnell Consulting Services and technical director for IGMA, says he sees at least “another generation” in the works. Meshulam sees fringe technologies, like radiant glass (for heating) and dynamic glazing, extending the reach of double-pane even further—by as much as a decade, he says.

In the meantime, nothing is certain.

In Japan, Sword says Pilkington has deployed a “hybrid” version of triple-pane IG that provides a twist on things, by including one airspace filled with krypton and another that’s vacuum sealed. Greg Kemenah, director of Guardian VIG LLC, says his company has started down a similar path.

In the end, experts agree that in order to justify their additional costs, new technologies must do more than just provide better performance. They must also prove to be more practical than good, old, double-pane IG.

Drew Vass is the editor of DWM magazine.

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DWM Magazine

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