New Windows....What does All those numbers Mean?

If you have ever purchased a new home, or replaced windows in your existing home you seen this sticker.

What does it all mean?


Windows are designed for certain geographical areas. This example is showing a window made for the middle to upper portion of the country.


The National Fenestration Rating Council (NFRC) in 1989 created standard metrics for comparing performance of windows. NFRC has set the format for those stickers for most of the time since then. It gives us four main numbers to pay attention to.


This particular window has a Energy Star Certified Rating for these areas.


Lets get into the Performance Ratings:

First is U-factor. (This window is showing a .27) Americans have gotten pretty comfortable with R-value, used to measure insulation, so it's a little unfortunate that we have to learn a new metric for windows, that is basically measuring the same thing. R-value measures resistance to heat flow; U-factor is the mathematical inverse (1 divided by R-value gives you U-factor), and measures the rate of that heat flow.

In a cold climate a good U-factor for a window is between 0.17 and 0.39. (That's between R-6 and R-2.5). Lower is better with U-factor--the opposite of R-value, when higher is better. The low end of that range is only achievable with higher-quality triple-glazed windows--windows with three layers of glass. In a hot climate, the only difference with U-factor is that it's better to look for an even lower figure. Look for a U-factor range in a hot climate of between 0.17 and 0.30, and again, lower is better. 


The next key number is solar heat gain coefficient (SHGC). (This window is showing .30) This is a number between 0 and 1, and it compares amount of solar heat that reaches the window with the amount that gets through to the inside. In a hot climate, or in a cold climate on west-facing windows with rooms that tend to overheat in low-angle afternoon sun, less solar heat gain is generally good, and this number should be low.

In cold climates on south-facing walls, a higher solar-heat-gain number is better--this supports "passive solar" heating. In this situation, an SHGC value of 0.42 to 0.63 is desirable, and higher is better. (In hot climates, look for values as low as 0.25.)

Unfortunately, due to the different low-e coatings used, higher SHGC values generally also come with somewhat higher U-factors. In other words, to get more solar heat gain you have to give up some insulation value.


Visible Transmittance: (this window showing a .51) There is no requirement for this number, and may not be shown on certain window manufacturer stickers.

The visible transmittance (VT) is an optical property that indicates the fraction of visible light transmitted through the window. This is separate from the Solar Heat Gain Coefficient (SHGC), since many modern windows include spectrally selective coatings that can allow different amounts of visible, infrared and ultraviolet light.

The NFRC's VT is a whole window rating and includes the impact of the frame area. Since the frame does not transmit any light, the VT may be lower than expected; however, this is done to be consistent with the whole window ratings of U-factor and SHGC. While VT theoretically varies between 0 and 1, most values among double- and triple-pane windows are between 0.30 and 0.70.


Air Leakage: (This Window is showing <0.3)Tiny cracks in the window assembly can add up to significant heat loss and gain, so it's worth looking for air leakage numbers . NFRC labels sometimes provide an air leakage rating (AL), although it is not required, and manufacturers often omit it. AL is expressed as cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the window assembly. Select windows with an AL of 0.30 or less. Lower is better.