Heat Transfer

• A window's primary job is to allow light into buildings without allowing heat or cold to pass between the inside and the outside. Light itself is energy and will carry a certain amount of heat inside, but most energy is lost by actual temperature exchange at the window, where air in the building loses or gains energy through the window. The more insulation qualities the window has, the better the barrier between the two levels of air will be, and the less easily they will be able to exchange heat.
  
  

Panes and Frames

• The panes of glass and the edges of the window are then two of the most important energy factors. Single panes of glass make fairly poor insulators, but double panes seal a space between indoors and outdoors, making the temperature-trading process take much longer, leading to greater efficiency. The most advanced windows use several panes of glass to make the exchange process even slower. Likewise, frame materials will decide how easily temperature can be exchanged at the edges of the windows. Traditional wood frames can warp and create spaces that allow air to escape, while newer materials like wood composite, vinyl, and fiberglass all keep heat transfer rates low.
  
  

Developments

• There are also several additional factors to consider when looking at energy efficient windows. Most windows have spacers, or strips of material that keep both panes a fixed distance apart. Made of metal or plastic compounds, these spacers can act as extra seals for the window. The glass itself may also have special properties, and some energy efficient window panes may be coated with a film that reflects different kinds of light, such as the hot infrared rays or the damaging ultraviolet rays.
  
  

Inert Gas Replacements

• A quad-paned windows with krypton glass, courtesy greendesignbuild.net
  
  The space between window panes may also be filled with different types of gas. While most windows just have air between their panes, the most energy efficient windows replace this air with an inert gas such as argon or krypton. These gases are harmless, but keep heat transfer minimal by slowing the exchange of energy between molecules. Air as a gas is fairly reactive and exchanges energy naturally with itself and its surroundings, but inert gases resist such changes and keep the temperatures in check.
  
  

Efficiency

• There are formulas designed to measure how effective windows are at saving energy, using factors to describes the area of the window and its heat flow properties. This formula is being expanded to include leakage rates and other window features. Windows with a good efficiency rating will always be more expensive, but will also do the best job at keeping temperature exchange minimal and saving energy.