Glazing is the glass in a window. There are three basic glazing options:  Single-glazed, Double-glazed and Triple-glazed.  The U-factor decreases as the number of panes of glass increase. Homeowners should usually expect an increase in energy efficiency to be accompanied by an increase in price because manufacturing costs generally go up as energy efficiency increases.

The most basic option available is a single-glazed window

This window has a single pane of glass and is the least energy efficient glazing option. Single-glazing replacement windows are rarely marketed today because of poor energy efficiency.  Ironically, a single-glazed window has become somewhat of a specialty product because it is no longer readily available.

Currently, double-glazed replacement windows are most commonly used.  These windows have two panes of glass with air between the panes to create a sealed insulating unit.  Argon or krypton gas may be used between the panes to increase the insulating performance as compared to air. These sealed units are most commonly between ¾ inch and 1 inch thick. The energy efficiency provided by double-glazed windows is appropriate for almost all regions within the United States.  In most circumstances a double-glazed window offers the best value when the considering the purchase price and the energy efficiency the window provides.

Triple-glazed replacement windows have three panes of glass that create two spaces with air or gas between the panes.  As with double-glazed windows ,argon or krypton gas may be used between the panes to increase the insulating performance as compared to air.  The additional insulating space increases the windows energy efficiency.  Triple-glazed windows are generally a better value for regions that experience extremely cold temperatures needing maximum energy performance and resistance to condensation.

For the best value in replacement windows, Mid South recommends double-glazed windows rather than triple-glazed for Knoxville and East Tennessee.

Double-glazed and Triple-glazed windows both use spacers to create the insulating space between the panes of glass. There are a number of different techniques used for spacers.  It is important to note that spacer technology  has a significant impact on the window's energy efficiency and may vary significantly from manufacturer to manufacturer.