Eliminating conduction and convection

Heat travels in three modes, and vacuum effectively eliminates two of them: conduction and convection. Researchers at Guardian Industries have recently unveiled a new breed of vacuum-glazed super glass with an incredible RI 2-RI 3 insulation rating. That means this new vacuum glass is as isolative as a thick insulated wall.

Guardian Industries has under development a revolutionary vacuum-glazing panel that provides a center-of-glass insulating value of R-12 to R-13. The glass has a very thin (250-micron or 0.25-mm) vacuum space. Using the same principle as a vacuum thermos bottle, these glass panels essentially negate two principal modes of heat transfer, paving the way towards windows that actually supply thermal energy instead of leaking it.

Heat travels in three modes, and the vacuum effectively eliminates two of them: conduction and convection. But the vacuum doesn’t eliminate radiation, so a low-e coating is critical to the unit’s performance. The total thickness of the GuardianVIG panel will typically be 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”.

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.

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.

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