If you are shopping for replacement vinyl windows, replacement wood windows or replacement composite windows, you're likely to hear a lot about energy savings.

Generally speaking, vinyl or composite windows rate better when it comes to thermal performance.

How can you compare one window against another? It's easy thanks to certain performance metrics that every window manufacturer must post on a label attached to every window they sell. You've seen energy performance labels on new refrigerators and new cars. Well, the government mandates similar data for windows.

The only problem with energy labels on replacement windows is that the terms and forms of measurement can be confusing.

Let's simply things.

First, every window comes with a U-Rating. The U-Rating measures how easily heat (or cold) passes through a window. The lower the U-Rating, the better. A window with a U-Rating of .30 is not as energy efficient as a window with a U-Rating of .28. How much more energy efficient? Although it is not exact, each 1/100th of difference in U-Rating is equal to a three percent difference in thermal performance. In other words, a window rated .28 is six percent more thermally efficient than a window rated .30.

Old, single-pane windows, or even double-pane windows that are more than 15-years-old often carry U-Ratings of .40 or higher. That's not good.

Ideally, you should stick with windows that are good enough to earn the Energy Star rating from the Department of Energy. That's a U-Factor of .30 or lower. At Wheaton Door and Window, we offer windows with U-Factors as low as .14. Now that's energy efficient!

The other numerical rating you are likely to see on a window's energy label is something called the Solar Heat Gain Coefficient or SHGC. This fancy term is used to describe how much of the sun's radiant energy is absorbed by the window's glass and frame and then radiated into your home. If you have a home that becomes intolerably warm in summer, you want a window with a low SHGC. Alternatively, if your house is always in shade, the SHGC is less of an issue. The SHGC is largely determined by the type and quality of glass in the window. Windows with lower SHGC's typically have coatings made of multiple layers of reflective material that filter out the sun's heat and reflect it back outside. This will help reduce faded carpets and furniture that often occurs in rooms with plenty of sunlight.

Always ask your window dealer for the energy stats on any window you are considering. It's an easy way of knowing what you are getting for your money.