Modern windows offer thermal efficiencies that would have been unimaginable not so long ago. Some of that improvement can be attributed to better-insulated frames, but most of it has to do with better insulated glazing units, or IGUs. A top-of-the-line triple-glazed unit filled with krypton or xenon gas may have a center-of-glass R-value of 10 or more. (The R-value of the completed window is partly determined by the frame and sash, and will usually be somewhat lower).
That kind of efficiency, however, comes at a cost. High-end IGUs are expensive in themselves, and because a triple-glazed IGU can be up to 1-1/2 inches thick, building a moveable window around one calls for a large, heavy frame and beefed-up hardware. That adds another layer of cost and helps explain why such glazing is rare on residential jobs.
But Guardian Industries — a major glass manufacturer based in Auburn Hills, Mich. — hopes to change all that. The company is developing a new line of vacuum-insulated glazing (VIG) units that it claims will equal or exceed the thermal performance of triple glazing without the weight and bulk.
Skinny vacuum. Vacuum-insulated glazing consists of two sheets of ordinary glass with an evacuated layer in between (similar to the walls of a Thermos bottle). Since the vacuum layer is only a fraction of a millimeter across, the completed unit is only about 3/16 inch thick, or about half the thickness of a conventional double-glazed window. The internal vacuum sharply reduces conductive heat loss, while a low-E coating helps slow the transmission of radiant heat.
A series of fine glasslike beads arranged in a grid pattern keeps the panes from touching each other. These “pillars,” as the manufacturer calls them, occupy so little surface area the conductive heat loss they introduce is negligible. A more significant source of potential heat loss exists at the edges of the units, where the two layers of glass are effectively welded together with a glasslike high-temperature material. Unlike the rubbery warm-edge spacers used in conventional multipane gas-filled units — which serve to thermally separate the inner and outer panes — the edge of a sheet of VIG constitutes a continuous thermal short circuit.
There are a couple of different ways to address that issue, says Guardian engineer Jeff Jones. The simplest method would be for window-makers to embed the conductive edge of the VIG more deeply into a well-insulated sash or frame. Or, he says, they could “create a hybrid where the vacuum glass acts like it’s one of the lights of a conventional IGU”; this would involve using a warm-edge spacer to isolate an additional pane of conventional glass from the VIG.
No pumping. Guardian’s new VIG will not be the first such product on the market. A Japanese manufacturer called NSG currently markets vacuum glass that has seen limited use in some North American commercial applications. The Japanese material, however, reportedly functions poorly in circumstances where there are large temperature differences between the inner and outer glass layers, because the resulting movement has been found to crack the edge seals. Although Guardian uses a similar sealing method, Jones claims that any tendency to crack has been “engineered out” of the company’s product. “Our testing shows that the seals are reliable at 70 to 90 degrees centigrade,” he says. “That’s a much bigger difference than you’ll find between the inside and outside of a warm house on a cold day in Bemidji, Minn.”
In fact, Jones says, the glass-to-glass edge seals of VIG may prove to be more durable than the warm-edge spacers in conventional IGUs. “With gas-filled windows, the area between the lights is under pressure,” he notes. Daily and seasonal temperature fluctuations cause the gas to alternately expand and contract, producing a “pumping” effect that tends to squeeze the insulating gas through the seals over time, and may cause the seals themselves to fail. “With vacuum glass, you don’t get that pumping because there’s no gas in there to expand and contract,” he says.
Someday soon. Stephen Selkowitz, program head of the building technology department at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, has been following the development of vacuum glazing for years. He says that Guardian “makes some interesting claims” but that the real issue is cost. “It’s a complicated manufacturing process. Making small quantities of vacuum glass in a research-and-development facility is one thing, but you don’t get economies of scale unless you commission a dedicated facility.”
Exactly when and where such a manufacturing plant might be built is something Guardian is keeping under wraps. But according to Jeff Jones, the company will be manufacturing VIG units in commercial quantities and shipping them to major window manufacturers sometime in 2011. Completed products will take a little longer, he says, given the need for window makers to adjust their own manufacturing processes and submit samples to the National Fenestration Rating Council for performance testing. But he says he is confident that manufactured windows with vacuum-insulated glazing will be commercially available by 2012.
As with any new product, consumer response is a major unknown, but focus groups conducted by Guardian suggest that one issue — the inconspicuous but clearly visible pillars separating the inner and outer panes — is unlikely to be a problem. “The majority of people don’t even notice them,” says Jones. “They could even become a status symbol, a way to point out that your windows have the newest and most efficient glazing available.”
