Most new homes being built today are built with double pane or insulated windows. These terms are generally used interchangeably. Double pane windows are composed of two pieces of glass joined by a spacer, which creates an “air space” between the two panes. The air space creates the insulating properties that make these units so energy efficient. The size of the air space determines the insulating value of the window (i.e. a unit with a 1” air space will be more efficient than a unit with a ½” air space). Additional options, such as “Low E” or tinted glass can also increase the energy efficiency.