Glazing Performance Metrics

Glass is everywhere in buildings—especially in large commercial buildings. It’s essential for views and daylighting but often a liability when it comes to thermal comfort and energy savings. Here are the metrics you need to know in order to understand their performance.

U-factor or U-value

U-factor (also called U-value) is the rate at which heat moves through a material for a given temperature difference, not including direct solar gain or air leaks. For most insulated glazing units (IGUs) the edges lose heat more quickly than the glass area, so it’s important to distinguish the “center-of-glass” U-factor from an overall unit U-factor. As IGUs get smaller, the edges have a bigger impact on the unit’s U-factor, but the center-of-glass U-factor is a property of the glazing that doesn’t depend on unit size.