Dual Pane Basics
Dual pane windows account for roughly 85% of all windows sold today. The reason is for their much improved energy efficiency over older single pane windows. The other category of windows still sold today that does not use two panes of glass is called a window insert, sometimes called a storm window, which are typically single pane frames that work with your existing windows.
More or less all remaining new construction and replacement windows sold today have two panes of glass as part of the insulated glazing unit or IGU. The IGU plays a huge role in the energy efficiency of modern windows sold in the past two decades.