As modern technology continues to take giant leaps forward, so does product complexity. Windows are no exception. Remember when they were available with only one sheet of glass mounted in either a wood or steel frame? If you broke the glass the repair could be done for under $5.

Window sales is one of America's biggest home improvement businesses. And triple-glazed windows are starting to gain momentum as a smart way to buy. And, although they sometimes can be a good financial investment, such is not always the case.

The Department of Energy states that in a new home the higher price tag that results from using triple-glazed windows instead of double-glazed can be saved in lower energy costs. Depending on optional features like Low-E and krypton or argon, the payback can take from four to eight years. This differs somewhat in an existing home where windows are being replaced. If replacement is due to damaged windows or remodeling, the payback is similar to that of a new home. But, if your existing double-glazed windows are in good condition it will take much longer to recoup your investment since a window can last 20 years or more. There is another factor to consider comfort increases as cold windows are eliminated.

A few things you should know before going shopping for windows:

Low-E stands for low emessivity. It is a coating that reduces the transmission of radiant heat by reflecting it. Some Low-E coatings are bonded onto the glass, others into the glass and still others onto a film placed between the layers of glass. In any event, Low-E coatings work to increase insulation. A double-glazed window with Low-E has about the same energy efficiency value as a triple-glazed one without Low E.

Inert gases such as argon and krypton radiate less heat than air and are used by many manufacturers to improve the energy efficiency of their windows. Although the use of an inert gas is not as effective as a Low-E coating, every little bit helps.

R-value is an energy efficiency rating. The higher the number, the better the insulation of the window. The top of today's market is R-8 with the average running between R-3 and R-6. Be careful here. An R-value can be expressed at the center of the window or as an overall measurement including the frame. This is important to know because some salespersons would have you believe that the performance of their window is based on the R-value measured at its center. For example: a window measuring R-3.7 in the center of the glass may actually be far less efficient R-2.85 or less when an overall measurement is made that includes the frame and the spacer between the glass. A metal spacer in a metal frame produces an overall rating of R-2. An aluminum spacer in a wood frame produces an overall rating of R-2.85. Remember, overall rating is the one that you should be looking for.

And don't be puzzled by U-value. U-value and R-value can be compared to the two different sides of the same coin. R-Value measures resistance to heat transfer the higher the better and U-value measures heat transfer the lower the better. To convert U-value to R-value divide the U-value into 1.

There are other important factors that need to be considered like air infiltration and how a window handles light, but a uniform method of testing has not yet been established. An organization called the National Fenestration Rating Council (NFRC) is currently working to develop uniform standards for measuring all windows.