Picture of a typical NFRC window label – Snagged from the NFRC website.
Almost everything you need to know about windows can be found on the NFRC label that comes on the window, and your window dealer should be able to furnish you with this information before you place your window order.
What you will find on the NFRC label
This is the energy efficiency rating of the entire window unit including the glass, jambs, and frames. U-Factor correlates directly to R-Ratings for insulation that we are all familiar with. If you divide the number one by the U-Factor you will get the R-Rating equivalent for the window. For example a U-Factor of .25 would be equivalent to an R-4 insulation rating. Notice that a lower U-Factor is better while a Higher R-Factor is better. This along with the air leakage rating is the main indicator of energy efficiency for the window, and is more important than particular construction features such as Low-E and Argon filled glass. Those factors help to determine the overall U-Rating, but other features not withstanding, for energy efficiency you want windows with a numerically low U-Rating above all else. Notice that even a very well rated window has a fraction of the insulation value of a standard exterior wall which will be from R-13 to over R-20.
Solar Heat Gain Coefficient
This rating (between 0 and 1) tells you how much heat from the sun is transmitted through the window. What you would like to see here depends upon the circumstances. The lower the number is the less heat will pass through the window – which is usually good. However there may be applications where you want solar heat gain in which case it would be a trade off between insulation value and solar gain. Here in the sunny south you probably want this number to be as low as possible to avoid heat gain during the Air Conditioning season.
VT is another number between 0 and 1 that indicates the amount of visible light that passes through the glass. The higher the number the more light the glass lets through. In this case more is better.
AL is an optional rating that some manufacturers use and some don’t. The higher the number the more air the window will leak. The value is supposed to represent cubic feet of air at a standard pressure differential. Obviously less air leakage is better.
CR values run between 0 and 100 with a higher value indicating higher resistance to condensation. This is also an optional rating.