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The ratings for glass are a bit more complicated than ones we are used to for insulation qualities. With insulation, we expect the higher R value to be a better insulator than one with a low. This is a fairly normal industry standard that has been around a good many years. The problem with window technology is that it is continuously changing and advancing. In recent years we have seen the gain of greater energy efficient models that meet a variety of goals for home owners in different climates. These ratings are not as simple as an insulator that just requires the manufacturer to increment the number to reflect effectiveness. In addition, more have been added. Understanding these values as a consumer and homeowner will ensure you do not end up spending money on features for windows you do not really need. While most windows sales representatives are not out to rip you off, you can still approach the purchase with a good amount of research and knowledge behind you.
Low E, Low E2 (squared), and Low E3 (cubed) windows are part of these advancements in window technology. The two key numbers for windows are generally the SHGC and U-Factor. The U-Factor is the insulating quality of the window and helps to keep you warm in cold weather. The SHGC is the reflective quality that helps to keep solar heat out and you cool in summers.
What we know as Low-E windows was one of the first advancements made in improving glass pane insulation. These types of windows feature a thin coating of metal on the glass that serves to reflect heat from the interior back in (U-Factor). As one of the first emission control features available; it's application was rather crude and inefficient. A layer of metal was simply attached to the molten glass while it was in production to give it that extra quality. Extensive scrubbing and abrasive cleaning on the window was able to damage the coating.
Low-E2 windows are an industry term to reflect the application of two coats of metal onto a window. This generation of windows used a technique called sputtering to help apply the metal solution. With the honing of this technique, silver flakes were introduced into the process to provide an even greater reflective surface for the windows to work with. The problem with this coating is direct exposure to air would cause the silver to tarnish like silverware would.
This made the dual-paned, inert gas filled windows an excellent vehicle for Low-E2 windows. The only problem with these windows is the inherent gas leakage one experiences and any weaknesses that might have existed in the spacers. Glass is made of approximately 75% sand so is a bit porous. Losing approximately 1% of the gas filling a year is not something to be considered a deal breaker. Inefficient spacers of the window can cause a much drastic loss instead. The quality of those should be double-checked if one decides to go with this solution.
Low-E3 windows are another industry term that simply reflects the application of three coats of metal onto a window. This style of window tends to be a great all climate window because it offers great performance in colder weather (U-Factor) and enhanced performance in summer (SHGC). The third coating layer provides great benefit for keeping unwanted heat out of a building but one can only see so much gain from applications for insulating qualities.
This is as close to an all-purpose window as one is going to be able to get. In the past, solar gain prevention windows operated largely with deeper tints. This product permits great performance coupled with easily visibility through it for homeowners.
This is mentioned due to the nature of the release of products. There is no universal trademarks when it comes to the Low-E products. If you are dealing with a company offering you Low-E4 windows, ask to see some performance charts compared against their other products. Some companies have released a "Low-E4 window" which is nothing more than Low-E window with a self-cleaning coating. They will normally advertise it as such, but the term Low-E4 is misleading because it does not follow the same patterns as Low-E, Low-E2, and Low-E3 already have.