What Type of Low E Glass Should I Use: Low E, Low E2 or Low E3?
Glass performance rating should be simplified. Most of us are familiar with wall insulation R-11, R-13, R-19, etc. and know the bigger the number, the better. With glass ratings, the lower the number, the better. Glass ratings generally consist of two key numbers: "U" Values, and Solar Heat Gain Coefficient (SHGC). "U" Value is a measure of how well a window will keep you warm in the winter, while SHGC is a measure of the window's ability to reflect heat away and keep you cool in the summer.
If you want to quit reading now, just note that you will want to see both of these numbers at .30 or less and you will meet the basic requirements for the tax credit of 2009. See the Department of Energy website for full details.
Low E windows
Low E, a first generation modern glass technology, made use of metal adhered to glass in a very thin layer to reflect heat back into the home. Original Low E glass was sometimes applied to single pane glass and had to be 'hard coated', which basically meant attaching a thin layer of tin to molten glass (which is composed primarily of silica sand). The hard coat is not as efficient as other Low E coatings, but can withstand a cleaning if it is not too abrasive.
Low E2 windows
Low E 2 (squared) is just a fancy term for 2 layers of reflective metal. This period of glass development was amazing. Every year, the coatings got more and more effective as sputter coats were developed incorporating silver in the process to develop micro fine reflective surfaces that greatly increased the energy performance of glass units. These soft coat layers are sandwiched between two panes of glass, so scratching or abrasive cleaners is not an issue.
One problem with silver is that it is shiny when new but can tarnish like silverware if exposed directly to oxygen. Thus, sealed dual paned units filled with argon are just the ticket for a long lasting Low E squared unit, with only the seal itself becoming the weak link in the system. Super spacers are the only truly effective seals to keep the argon between the panes of glass.
About 1% of the argon will still permeate through the glass itself (remember, the glass is made of 70% sand). A 1% degradation is one we can all live with, but the 10% average annual loss with metal spacer systems is questionable at best. While we all like to make things cheaper, this may not be the best place to do it, as the loss of argon gas degrades the window performance and degrades the silver coatings of Low E.
Low E3 windows
Low E 3 (Cubed) is a term for three layers of silver. There is a law of diminishing returns and Low E cubed offers very little gain in the area of keeping warm in the winter (U Value), but makes great strides in offering tremendous gains in summer performance (SHGC). Low E cubed effectively makes a single window unit that is equally at home in all climates. It insulates very well in a cold area like Nebraska, while performing great for an Arizona summer.
Until recently, people in northern areas worried about cold, while those in the south worried about heat. Then along comes the tax rebate for a window with a U Value and a SHGC of .30 or less, and glass companies scrambled to meet the challenge. Window companies across the country have been forced to scramble and get their windows retested with the new glass and have created sudden demand for rating companies like the NFRC. This is as close to a 'one window does it all' as has ever existed. In the past, solar tints were used to control the heat, but were very dark.
This new technology allows people to control the heat and still keep a viewable window without the need for tints. In summation, it makes sense to choose Low E cubed glass with a super spacer. Do a search for Low E 366 to see some nice Cardinal Glass Co. graphs of performance. Another nice thing about using high performance glass like this is that it is good for the environment by saving you money on your heating bills while making your home more comfortable to live in.
Beware that anyone can create a name for a glass and trademark it. 'Low E 4' is out in the market and is basically Low E 2 with a self cleaning coating. Also available is ThermalpermE, which is a renamed generic Solarban. The largest glass companies will keep you up to date on what is really available. Visit the PPG Glass site or Cardinal Glass site to keep abreast of what is the latest and greatest.
By Tom Gion
