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Vacuum Insulation Glass: How VIG works

The heat transfer in conventional double and multi-glazing systems is caused by the following mechanisms:

A third mechanism comes into play with vacuum insulation glass (VIG), namely thermal conduction via the supports between the panes. The insulating efficiency of a glazing system can be increased by reducing the effect of each of these mechanisms. Non-evacuated glazing systems are filled with special gases (e.g. argon or krypton) to lower the thermal conduction of the gas between the panes. The thermal radiation between the panes can be considerably reduced by applying IR-reflecting coatings (low-ε coatings). Replacing the air filling with krypton and applying a low-ε coating with ε = 0.04 decreases the Ug-value of a double-glazing system by about 3 W/(m²K) to approx. 0.9 W/(m²K). Lower Ug-values can be achieved, for example, by added further insulating layers, such as with triple-glazing. The considerable disadvantage of triple-glazing is the greater weight and higher system thickness. A different approach is used for vacuum insulation glass: The effective thermal conductivity of the filler gas is dependent on the gas pressure and the gap between the panes. The graph below shows the thermal conductivity coefficient of a 1 mm wide gap filled with air in dependence of the gas pressure. Gas pressures of <10 - 3 mbar are needed to significantly reduce the thermal conduction of the filler gas.


Theoretical Ug-values of approx. 0.15 W/(m²K) can be realized with negligible filler gas thermal conductivities and efficient low-ε coatings. Unfortunately, such evacuated systems require supports between the panes, which represent thermal bridges which cannot be ignored. To ensure mechanical stability, above all, the intervals between the supports must be kept to a minimum, which is not optimal from a thermal point of view. Experiments have shown that Ug-values of < 0.5 W/(m²K) for 9 mm thick vacuum insulation glass with thermally optimized supports are possible.

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