The effort focuses on developing specifications and test methods to establish a performance baseline that allows products to be evaluated on a level playing field and provides a reference point for code bodies and local code officials. The most recent and exciting example of such development is the emergence of Vacuum Insulated Glazing (VIG), and work proceeds to enable its widespread commercialization in the near future.The VIG ConceptThe current prevailing insulating glass technology replaces air in the space between two panes of glass with a lower thermal conductivity gas such as Argon, Krypton or Xenon.Argon has a thermal conductivity 67 percent that of air, Krypton has about half the conductivity of Argon (35 percent that of air) and Xenon has about 39 percent that of Krypton (about 22 percent that of air). Krypton and Xenon are, however, very expensive.
The most thermally efficient gas fill would be no gas at all—a vacuum –in which the space between surfaces two and three is evacuated to a minimum of less than one millionth of normal atmospheric pressure. At that level, there is no conductive or convective heat exchange between the lites of glass, thus lowering the U-factor.
Gas-filled IGU thickness is a compromise between maximizing insulating value and the ability of the framing system used to carry the unit. This trade-off does not apply to Vacuum Insulated Glass (VIG) as heat transfer due to internal convective currents is eliminated, leaving radiation losses and conduction through the edge seal to deal with.
A good low-E coating reduces radiation heat loss. Depending on the type of low-E coating, dual-pane VIG windows can exhibit a U-factor of 0.20 or less (~ R-5 or greater, in the currently popular terminology for super-insulating windows per DOE’s “R-5” initiative) while maintaining a high SHGC.
But edge seals pose a bit more of a challenge.
The two lites are sealed at the edges, with glass spacers as thin as a human hair keeping the two lites from touching. The airtight vacuum seal around the IGU edge must be maintained for the life of the window, through manufacture, transportation and installation, as well as normal operation, wear and weathering.
In addition, a grid pattern of fine glasslike beads keeps the panes from touching each other. These “pillars,” as the manufacturer calls them, occupy so little surface area that their conductive heat loss is negligible. They are small enough that they are not visible from more than a few feet away. Because they are inconspicuous, focus group research indicates that this is not likely to be a drawback to acceptance.
Potential Energy Game-Changer
On-going R&D indicates that VIG is some five times better at insulating a home than conventional double-pane glass and can enable whole-window R-values of 10 or greater.
Although a top-of-the-line triple-glazed unit filled with krypton or xenon gas can also currently claim a center-of-glass R-value of 10 or more, VIG units promise to equal or exceed the thermal performance of triple glazing with less weight and bulk.
Since the vacuum layer is only a fraction of a millimeter across, VIGs have the advantage of being thin (0.20–0.43 inch) or about half the thickness of a conventional double-glazed window, and are thus suitable for many facade designs, including retrofits.
Wrapping Up Engineering Issues
Commercialwindows.org – a site that is owned, operated and maintained by the Efficient Windows Collaborative (EWC) with the support of the U.S. Department of Energy's Windows and Glazing Research Program – notes that evacuated window assemblies have presented a number of engineering problems.
One major issue is the structural requirement to resist normal air pressure and variable pressures caused by wind and vibration.
Another concern is that a 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 insulating warm-edge spacers used in conventional multi-pane gas-filled units, the edge of a sheet of VIG constitutes a continuous thermal short circuit. The edge seal and spacers need to be perfected and proven to function for more than 30 years.
One solution may be a hybrid in which the vacuum glass acts as one of the lites of a conventional gas-filled IGU. A warm-edge spacer would isolate an additional pane of conventional glass from the VIG.
The VIG glass seal is also rigid, and will experience increasing stress with increasing temperature differential across the large window-sized panes of glass. An earlier version of VIG is said to function poorly with large temperature differences between the inner and outer glass layers, because the resulting movement has been found to crack the edge seals. This stress may prevent vacuum glazing from being used when the temperature differential is too great. One manufacturer provides a recommendation of 35 degrees C (63 degrees F) difference or less.
The real hurdle to early commercialization may well be the cost. Stephen Selkowitz (Lawrence Berkeley National Laboratory) notes that VIG production “is 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.” Such manufacturing plants are in the early stages of coming on line.
In addition, window makers must adjust their own manufacturing processes to accommodate VIG and submit samples for NFRC performance testing.
The IGMA VIG Task Group (Chair: David Cooper [Guardian]) has taken on multiple objectives. Currently in process among these is the development of a Technical Bulletin which will provide an overview of VIG and its use. The bulletin will contain:
A brief history of VIG, including early patents, University of Sydney work undertaken by Dr. Collins and recent innovations
General Information on the technology, including glass and coating, array of pillars or spacers, hermetic edge seal, hermetically sealable evacuation ports, the importance of vacuum stability and finished product characteristics (size range, testing, shipping considerations)
VIG Development Influencers, VIG Use and where it can be used
Main benefits of VIG, potential limitations and VIG glazing installation
IGMA Executive Director Marg Webb reports that the first ballot on the document should be circulated this July, with the objective of publication late this fall.
Following issuance of the Technical Bulletin, the VIG Task Group plans to:
Develop a VIG test standard (future)
Develop a VIG fabrication and certification standard (quality, fabrication and certification aspects)
Add VIG to the IGMA Glazing Guidelines
It will be a revolutionary process to see windows on the market capable of thermal insulating properties that are about the same as those of a brick and plaster wall.