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Buying the right energy-efficient windows for your own particular needs goes beyond considering just the R value of the glazing system.  

The window frames also should play an important role in the buying decision, as should the location of the house and where in the house the new windows will be installed. 

For example, cutting heat loss is important in Minnesota, but not so important in Florida. Similarly, a window on the north side of a house in a cold climate should be designed to save energy. Those on the south side should save energy and allow as much solar energy as possible to pass through. Here are some points to consider when shopping for new windows: 

Energy ratings – Most manufacturers provide both the center-of-glass and the entire window energy ratings. Obviously, the entire window R value is a better indicator of performance. Don’t be confused if you see a U value alongside the familiar R rating. They are different sides of the same coin. R values measure resistance to heat transfer – the higher the better. U values measure heat transfer – the lower the better. To keep everything straight, convert U values to R values by dividing the U value into 1. 

Shading coefficient: This rating tells how much solar energy a window captures. A single pane of glass has a shading coefficient of 1. A blocked window would be rated at 0. So, if reducing cooling costs is important, pick a window that has a low shading coefficient. Tinted windows are good choices, but low-e technology designed for warm climates can also produce low shading coefficients while providing clear glass. 

Handling light: Besides letting heat energy pass through, windows also allow light into our homes. Not all windows do this equally. Clear double-glazed windows allow about 77 percent of the visible light to pass through. Low-e glass allows about 70 percent and a high-R window, around 62 percent. Part of the light is in the form of the ultraviolet radiation that’s responsible for the fading of carpets and furniture. You’ll find windows that allow everything from over 50 percent of the UV radiation through, to windows that let less than 1 percent through. 

Air infiltration: This is important in all climates. Windows should be well constructed and allow a minimum of air infiltration. The design of some types of windows make them tighter than others. Casement And awning windows are tighter than double-hung windows, for example.  

A rating of .02 or .03 is very tight; a rating of .5 is loose. These ratings apply to the window itself, not the actual installation. Stopping leaks around a window once it’s in the wall is the responsibility of the installer. 

Buying windows can get confusing. The trick is to decide what each window should do and then buy the system that best meets those goals.