Double glazing is an energy-saving feature which provides extra insulation by adding an additional layer of glass to the windows (normally with a pocket of air between the layers). The feature is only listed for structures where at least 80% of exterior-facing windows are double glazed. Any buildings with 3 or more layers of windows can also be labeled as double glazed.

This should not be confused with a double skin facade, which normally requires two totally separate window framing systems separated by a much greater distance than the panes in double glazing.

The threshold is at least 80 percent of the exterior-facing windows, and three or more layers are included. The separate double-skin facade arrangement is not established merely by multiple panes within a window.