Double (or more) glazed windows: Multiple layers of glazing is one feature you must consider for energy saving windows. While it is not the only way to save energy, until the 1980s it was the primary way manufacturers improved the energy performance of windows. Adding additional layers of glazing increases insulation very well. Double glazing insulates almost twice as well as single glazing. Adding a third or fourth layer of glazing results in further improvement. Some of these windows use glass only; others use thin plastic films as the inner glazing layers, regardless of the material more glazing equals more energy savings.
 Any window with multiple panes has air space in between the panes, one of the things you want to look at is the thickness of the air space. With double-glazed windows the air space between the panes of glass has a big effect on energy performance. A very thin air space does not insulate as well as a thicker air space because of the conductivity through that small space. In recent years many window manufacturers have increased the thickness of the air space in their double-glazed windows from ¼" to ½" or more. But, remember that too wide is just as bad as too thin. If the air space is too wide, convection loops between the layers of glazing occur. So, know that beyond about 1", you do not get any further gain in energy performance with thicker air spaces.
 Look for low-conductivity gas fill. What is the point of this? Well, by substituting a denser, lower conductivity gas such as argon for the air in a sealed insulated glass window, heat loss can be reduced significantly. Argon gas is better than just using air. Most major window manufacturers offer argon-gas fill as an option in their most popular windows, so be sure to ask for it.
 Tinted glass coatings. Tinted glass and tinted window films have long been used in commercial buildings to reduce heat gain through window, as well as in cars, so why not use them in your home? Obviously you do not want dark tints, but lightly tinted windows are becoming popular. This glazing type reduce the solar heat gain without reducing visibility as much as older tinted glass and films.
 Be sure to find windows with low-e coatings. More than any other single improvement, the invention and commercial development of low-emissivity (low-e) coatings in the 1980s revolutionized window technology. Thin, transparent coatings of silver or tin oxide permit visible light to pass through, but they effectively reflect infrared heat radiation back into the room. This reduces heat loss through the windows in the winter. So, if you live in a climate that has cold winters, this is not a feature you want to pass up. There are a variety of low-e windows available for different climate zones and different applications. Low-e windows with high solar heat gain coefficients are appropriate for northern climates where passive solar heating is advantageous, while "southern low-e" windows with low heat gain coefficients are appropriate in milder climates where summer cooling is more significant than winter heating.
 Another great feature for energy saving windows is quality edge spacers. The edge spacer is what holds the panes of glass apart and provides the airtight seal in an insulated glass window. Look for edge spacerrs that made of thin-walled steel and have a thermal break, do not look for aluminum. You may try space edgers made of silicone foam or butyl rubber. Generally, better edge seals are a low-cost option when ordering windows, and are worth considering because of the increased energy savings.