What is the g-value of a sealed unit?

The g-value (also called solar factor or solar heat gain coefficient) is the proportion of the sun’s heat energy that passes through the glazing into the room, expressed as a figure between 0 and 1. A g-value of 0.6 means 60% of the available solar heat gets through. It matters because it’s one of the three inputs to the Window Energy Rating — solar gain is the “positive” that’s offset against heat loss and air leakage. A higher g-value means more free solar heat in winter, which lifts the energy rating; but on large south-facing glazing it can also contribute to summer overheating, where a lower g-value (solar-control glass) is preferable. Standard Low-E double glazing typically sits around a g-value of 0.5–0.63 depending on the coating. For most jobs the standard unit is fine; on big south- or west-facing elevations it’s worth discussing solar-control options.