Understanding Replacement Window Rating Terms

If you’ve ever looked into replacement windows, you’ve likely noticed each one has a sticker with lots of numbers, terms, and acronyms. Those stickers are there to help you compare the windows in an apple to apples fashion. The window companies don’t get to randomly decide what they put on the sticker, there are formulas that determine what numbers go next to those terms. The National Fenestration Rating Council® certifies the accuracy of the numbers on different windows.

Although each window may have many different components from the frame material to the number of panes of glass, and if they are filled,  and how they open, each window has the same energy performance ratings. The information is displayed with the same information in the same order to help you easily compare the windows. They are:

U-Factor (or U Value)

This is one of the required measurements on any new or replacement window. The U-Factor describes how resistant to heat flow a window is. This number ranges from 0 to 1 with the lower number indicating a more energy efficient window. In our area, ENERGY STAR recommends a U-Factor equal to or less than .30.uvalue

SHGC or Solar Heat Gain Coefficient

Much like U-Factor, this is on all windows and has a rating of 0 to 1 with the lower the number, the more efficient the window. It refers to the amount of heat that comes in from the outside from the sun – the solar heat absorbed. A number less than or equal to .40 is accepted in our region.

The rest of the performance ratings are optional but the more information the window manufacturer provides, the better you can compare various windows.

VT or Visible Transmittance

This rating is one of personal preference. If you’re looking for a window that lets in the most light, then you want a window with a number as close to 1 as possible. This is on a scale of 0 – 1 with the higher number letting in more light.

Air Leakage

This one is usually written out and fairly self-explanatory. Rather than the 0 – 1 scale, this is expressed in cfm/ft2. When you’re comparing windows, the one with the lower number has less leakage around the window.

Condensation Resistance

This number is determined through testing the window under various humidity levels and temperatures. The condensation resistance rating is between 1 and 100. The higher the number, the less likely the window will form condensation under normal circumstances. This rating is found on the fewest windows.

Now that you better understand the terms used on replacement windows, it will be easier to select your new windows. Cooley Roofing and Construction is ready to help you with you replacement windows, new siding, or roof repair or replacement. Call us at (336) 766-6488 to schedule your appointment.