Window Materials & Assemblies: Emerging Technologies
Insulation-filled Glazings | Evacuated Windows | Smart Windows | Building Integrated Photovoltaics
Evacuated Windows
The most thermally efficient gas fill would be no gas at all-a vacuum. A number of researchers around the world have been pursuing the development of insulating window units in which the space between the panes is evacuated. If the vacuum pressure is low enough, there would be no conductive or convective heat exchange between the panes of glass, thus lowering the U-factor. A vacuum glazing must have a good low-E coating to reduce radiative heat transfer-the vacuum effect alone is not adequate. This principle has been used in the fabrication of thermos bottles for many years, with the silver coating serving as the low-emittance surface.
Evacuated window assemblies present a number of engineering problems, however. One major issue is the structural requirement to resist normal air pressure and variable pressures caused by wind and vibration. There can be large thermal stresses between large, window-sized panes of glass. A thermos bottle resists these forces easily because of its strong curved shape, but the large, flat surfaces of a window tend to bow and flex with changing pressures. In prototypes, minute glass pillars or spheres have been used to maintain the separation between the panes. The pillars are very small but are somewhat visible, reducing the window clarity.