Replacement window energy ratings are located on the NFRC Energy Star sticker occumpanying any energy star window and door. Some companies push that their window has great R-value, others a great U-factor, and in the southern states, Solar heat gain coeffienct. So what do these energy ratings mean? Truth is, these numbers are important for different people in different places. Most of these numbers can be found right on the NFRC label adorned on every major brand of window in America. 
What’s R-Value? R-Value is a measurement of the heat retention based on the surface area of the entire window. Many companies say that r-value is the most important number when factoring a window, but the fact is a window with a hole in the middle of the pane of glass can have a better r-value than a top of the line triple pane window. Most windows with high R-value have large bulky frames, which contribute the most when talking about r-value (expressed in area). It takes a very bulky and heavy window to get to an R-value of 7.0, which is equivalent to about a half a foot of fiberglass insulation. The higher the R-value the better. Again, R-value has no bearing on how well a window stops air or heat. R-value is not required by the National Fenestration Ratings Council to be on an NFRC label. What’s U-Factor? U-Factor is a measurement of the ability of a window to stop air and heat from flowing in and out of the window. The number is measured on a decimal scale between 0 (best) and 1 (worst), unlike R-value which is measured with the higher number the better. U-factor in most professional’s opinions is the best way to measure the performance of any window (besides the actual installation, including insulation around the window). To the right you can see the NFRC label for the window we install. Which is a better indicator? In the above picture you can see a side by side comparison of the difference between a window with an insulated frame and a non insulated frame. The channel of white you see in the picture on the left is where air and heat can move through the frame. On the right you can see that with a insulated frame, air cannot move through. A frame with no insulation could have a much lower U-factor but not necessarily a lower R-value. Keep in mind there are other factors besides U-factor (such as structural integrity), but U-factor is usually the best indicator of how energy efficient a window is. So what about Solar Heat Gain? 
Although not as big of a deal in temperate climates, in the Southern U.S. a heavy emphasis is placed on Solar Heat Gain Coefficient (also on the NFRC label). This number expresses how much heat gets transferred through the window in direct sunlight. The lower the heat gain coefficient, the better in terms of not turning your home into one big magnifying glass. Like U-factor, it is expressed in a number between 0 and 1. In places like Florida and Arizona, SHGC is a hot button amongst installers and consumers.
