You’re investigating replacement windows and see many window rating terms on the label, but what do they mean? If the primary purpose in replacing your windows is to save money, understanding these acronyms and codes is essential. Today’s blog aims to help the consumer decipher those terms and abbreviations.


When looking at a new window, the u-factor tells you the window’s resistance to heat flow. The lower the number, the better the insulative properties of the window. A highly insulative triple pane window might have a U-factor as low as .15 while a mid-range double-pane window might be around .4. Some window manufacturers confuse the issue by providing two U-factor numbers, one for the whole window and one for the glass. If this is the case, the whole window number will be higher than the center-of-glass rating. The U-factor range is 0 – 1.


The SHGC is the solar heat gain coefficient. Again, this is another measurement where lower is better and the range is from 0 – 1. Solar heat gain coefficient refers to the solar radiation that becomes heat inside a home. It is essentially the solar heat absorbed through a windows. Like U-factor, the National Fenestration Rating Council or NFRC determines this number.


The VT is the visible light transmittance of a window. Number of panes, glazing and tinting all impact the amount of light that comes through a window. The higher the VT number, the more light comes in. If you are looking for a bright space, choose windows with a number closer to 1 as the range here is also 0 – 1.

Air Leakage

Air leakage is probably the most self-explanatory of the terms used. It is the amount of air that moves around a window. You should notice a cfm/ft2 following the number. This is the exception to the 0 – 1 rating scale as this is expressed in cubic feet per minute per square foot of window frame. When comparing windows, ones with lower ratings is better than ones with high air leakage.

By comparing these numbers on various windows you are considering, you should have a better idea of why a window is more expensive and if it will save you more money. For homeowners in the Dallas-Fort Worth areas, the recommendations are to choose windows with a U-factor equal to or less than .3, a SHGC equal to or less than .25 and an air leakage .3 or less. There is no recommendation of the VT as this is more a matter of personal choice.

Affordable Replacement Window Systems offers many windows that meet the recommendations by the Efficient Windows Collaborative. If you’re considering replacement windows, give us a call and we’ll provide you with a no obligation consultation and will further explain any other window rating terms. Let us help you have a more comfortable home today! Call (817) 264-6509 to schedule your appointment or fill out our easy to use contact form and we’ll give you a call.

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