This has very little insulating value with (approximately R-1). It provides only a thin barrier to the outside and can account for considerable heat loss and gain. One pane of glass used to be the only alternative for windows, but not anymore. Double pane windows have become extremely well-known in new development and as replacement windows throughout the years, due to their energy efficient properties and other benefits, for instance sound reduction.
Double pane windows are improved insulators, keeping warm air in and cool air out. What makes them such successful insulators? Part of the answer lies not in the glass itself, but in the space between the two panes. It creates a stabilzed air space or buffer between the exterior and interior of your home.