February 27, 08
Revolutionary R-12 Vacuum Glass Turns Windows into Walls

Guardian Industries, one of the world’s largest architectural and automotive glass manufacturers, with 19,000 employees in 25 countries, has under development a revolutionary vacuum-glazing panel that provides a center-of-glass insulating value of R-12 to R-13. The glass—Guardian VIG (for vacuum-insulated glass)—has a very thin (250-micron or 0.25-mm) space evacuated to 10—4 torr (for reference, thermos bottles typically have a much harder vacuum of around 10—6 torr) between two layers of glass, one of which has a low-emissivity (low-e) coating. Guardian is currently producing the vacuum glazing on a limited basis for testing and hopes to roll it out commercially by the end of 2009.

Heat travels in three modes, and the vacuum effectively eliminates two of them: conduction and convection. But the vacuum doesn’t eliminate radiation, so the low-e coating is critical to the unit’s performance, according to Scott Thomsen, the chief technology officer and group vice president of Guardian’s Science and Technology Center. The type of low-e coating also has a huge effect: a hard-coat (pyrolytic) coating yields only R-2 to R-3 insulation, and standard low-e coating yields R-4, Thomsen told EBN. To achieve R-12, Guardian uses its more advanced ClimaGuard Low-E glass, which has two deposited metallic layers like Cardinal’s LoE2. Energy performance testing of the Guardian VIG panels has been done at the University of Waterloo in Ontario.

The total thickness of the Guardian VIG panel will typically be 0.26 to 0.43 inch (6.5—11 mm), depending on the glass used. This is far thinner than conventional insulated glass units, which require triple glazing and multiple low-e coatings to achieve comparable energy performance. The vacuum pulls the layers of glass together, so Guardian uses tiny, low-conductivity “pillars”? several inches apart to maintain the spacing between the layers of glass. These are almost invisible, though “if you know to look for them, you’ll find them,”? said Thomsen. The spacers will be most noticeable at night, he told EBN. In focus-group testing, Guardian found that only about half of the participants noticed the spacers.

For even better energy performance, Guardian could fabricate the VIG panel into a glazing unit with a spacer and a third layer of glass, adding R-1 to R-5 to the overall insulating value. The company is testing such configurations as well as options that would add a layer of thin-film photovoltaic cells. The Michigan-based company began researching vacuum glazing technology in 2000 and holds 42 patents on vacuum-insulated glass. Earlier efforts with vacuum-insulated glazing in Australia ended when university researchers couldn’t get the projected manufacturing cost below about $15 per ft2 ($16/m2), according to Thomsen.

Guardian made progress but by 2003 still had been unable to get the cost below $6—$8 per ft2 ($65—$86/m2). Development of several new materials and automated manufacturing processes have more recently enabled further reductions in cost. Guardian is in discussion with two major U.S. window manufacturers about use of this glazing.

When asked about the technology, Stephen Selkowitz, head of the Building Technologies Division at Lawrence Berkeley National Laboratory, was impressed. If the company can achieve an R-12 center-of-glass insulation level at a reasonable incremental cost, “that’s huge,”? he said. “This performance level would convert most windows in heating climates into net energy suppliers, providing more energy to the home via passive solar gain (even facing north) than the window loses,”? he told EBN. “If you could convert every window [in the U.S.] to this performance level, you would save homeowners about $15 billion each year.”