A multitude of options exist for windows in the marketplace. Some are more appropriate than others for a particular climate or location. The different ratings that one generally looks for reflect different aspects of the window's efficiency. Energy saving features are what help to determine the overall performance and ratings of a window. Some of the most prominent are Low-E coatings and multiple-panes.
A great innovation in window efficiency came in the form of the process of Low-E coating a window. This process saw the window glass bond with flakes of metal during the production process. In turn, it created a type of glass that was better able to deflect heating and cooling. The energy saving feature allowed home owners to create and maintain a much more comfortable atmosphere in their home.
In time, the process grew to include additional coatings. Now we have Low-E, Low-E2 (squared), and Low-E3(cubed) coatings available. The thing to understand about multiple coatings is that they operate under the rule of diminishing returns. That basically means that the more coats you add, the less effective the later coats become. In the case of Low-E coatings, one will find that their performance offers little gain past the initial coat in keeping warm in the winter. During the summer however, these windows offer tremendous gains in sealing cool air into the house.
A new product being offered by some companies is referred to as Low-E4. This type of window features a self-cleaning coating with the equivalent of a Low-E coat on it. There is no industry standard for who can name what. In this case, you go from efficient (E), to more efficient (E2), to most efficient (E3), to self-cleaning efficient (E4). Do not get sold on Low-E4 windows being more efficient than Low-E3. They are not.
One of the main energy saving features that was developed was the installation of additional panes of glass in the window. These are available in either double or triple paned models that drastically increase the efficiency of the window by diffusing more energy on either side. An inert gas filling of Krypton or Argon further insulates the window and prevents the energy from passing.
Sunlight energy strikes the exterior pane and begins to heat it up. Once it is heated to the point where it can no longer contain any more heat, it begins to pass the heat through it. A gas insulator dampens the transition and forces much of the heat back out of the window instead of allowing it to the interior pane where it can then transfer inside.
The system works the same from the inside. Hot or cold energy strikes the interior pane, once it is at a transferable point the inert gas acts as an insulator to prevent it from exchanging to the exterior pane. This allows the respective weather and temperature to stay on their given sides of the window.
Triple-paned inert gas filled windows mirror this process except they feature an additional pane. The additional pane and gas creates an even tighter buffer. In most temperate and non-extreme climates, double-paned are an appropriate choice. In areas of bitter cold or blistering heat, triple-paned may be a better choice.
Well constructed windows will generally only have one weak point in their insulating qualities that can skew their energy saving. That is the seals that hold surround the glass panes. One can expect to lose approximately 1% of the gas filling per year through the glass itself. Remember, glass is made primarily of sand which has some porous qualities to it. A faulty seal on the window can cost you 10% of the gas filling or more per year; which is not acceptable for energy savings or the investment.