In terms of energy efficiency, windows are one of the biggest contributors to heat loss from buildings. However, a new window technology being developed by Guardian Industries could allow for windows that can provide insulation values comparable to a standard insulated 2×4 stud wall, with a new double-pane glass and a vacuum between the panes.
A thermos keeps hot beverages hot and cold beverages cold by separating the contents from the outside world with a vacuum. Heat is conducted by three modes, conduction, convection, and radiation. A vacuum prevents conduction and convection, and a reflective coating serves to reflect radiated heat back where it came from. The Guardian VIG (vacuum-insulated glass) works the same way, with a vacuum between two panes of glass, and a low-E coating to prevent radiant heat from escaping.
Ordinary single-pane window glass is not much better than an open window when it comes to insulation value. Even a good quality double-pane window only has an R-value of 3 or 4, typically. A typical 2×4 stud wall has an R-value of 12 or better. Superinsulated houses with thick wall construction and very high R-values will often use triple glazing, and will minimize the size of the windows in order to keep the efficiency of the system as high as possible and reduce the amount of energy lost through the windows.
The new glass provides a vacuum space between two panes of glass. To keep the two sheets of glass from being drawn together by the vacuum, low thermal-conductivity spacers are placed in the space between the two panes. (These are the small dots that can be seen in the photograph.)
While the vacuum is only about 1/100th as strong as what is typically found in an ordinary thermos, it is still far better than standard double pane glass in preventing heat loss from conduction and from convection. The only other glazing systems I have come across with close to this level of insulation value have been nanogel-filled windows, but those are just translucent, and do not allow clear vision through the glass.
The manufacturer, Guardian Industries, is reportedly hoping to have this glass commercially available by the end of 2009. More importantly, while other researchers have been exploring the idea of vacuum glass for several years, Guardian is expecting to be able to produce this glass at a reasonable cost over conventional glass.
The head of the Building Technologies Division at Lawrence Berkeley National Laboratory, was quoted as saying, “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 looses.”
via: BuildingGreen.com
Image Sources: Passive House Institute and Bulding Green.com
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