Insulated glass

Thermal

This trade-off does not apply to vacuum insulated glass (VIG), or evacuated glazing, as heat loss due to convection is eliminated, leaving radiation losses and conduction through the edge seal and required supporting pillars over the face area. These VIG units have most of the air removed from the space between the panes, leaving a nearly-complete vacuum. VIG units which are currently on the market are hermetically sealed along their perimeter with solder glass, that is, a glass frit (powdered glass) having a reduced melting point is heated to join the components. This creates a glass seal that experiences increasing stress with increasing temperature differential across the unit. This stress may limit the maximum allowable temperature differential. One manufacturer provides a recommendation of 35 °C. Closely spaced pillars are required to reinforce the glazing to resist the pressure of the atmosphere. Pillar spacing and diameter limited the insulation achieved by designs available beginning in the 1990s to R = 4.7 h·°F·ft2/BTU (0.83 m2·K/W) no better than high quality double glazed insulated glass units. Recent products claim performance of R = 14 h·°F·ft2/BTU (2.5 m2·K/W) which exceeds triple glazed insulated glass units. The required internal pillars exclude applications where an unobstructed view through the glazing unit is desired, i.e. most residential and commercial windows, and refrigerated food display cases. VIG equipped windows, however, under-perform due to intense edge heat transfer.