How is Window Performance Measured?

Have you just about had it with your old, drafty windows? Is this the year you’ve decided to finally replace your windows? Replacing your windows with new ones comes with lots of benefits, including an energy efficiency boost, better ventilation, and better quality of light in your home. The National Fenestration Rating Council certifies and labels windows (as well as doors and skylights) on their performance and energy efficiency. When you’re shopping for new windows you’ll see these ratings on the NFRC label. In this week’s blog, we’ll talk about how to read this label to make sure you’re making an informed decision on your new windows.


Learn how window performance is measured, in our blog.

NFRC Window Performance Measurements


Heat Gain and Loss


The first three properties on the label have to do with how the window performs with regard to heat gain and loss. Windows gain and lose heat in three ways:

  • Direct conduction through the glass.
  • Radiation of heat from the sun into the house, and out of the house from objects in the house.
  • Air leakage through and around the window.


This is “The rate at which a window, door, or skylight conducts non-solar heat flow.” The takeaway here is “The lower the U-factor, the more energy-efficient the window, door, or skylight.”

Solar Heat Gain Coefficient

The SHGC tells us how much radiation is admitted through the window and released as heat in the home. The lower the number, the less heat is transmitted. However, this doesn’t necessarily mean you want a low SHGC. For example, because a higher SGHC means the window allows more heat in, you can allow more solar heat inside in the winter, which could reduce your heating requirements. In this case, the climate you live in will play a major factor in choosing an SHGC rating.

Air Leakage

This quantifies how much air the window lets in relative to a specific pressure difference across it. The lower the rating, the less air leakage.

Sunlight Transmittance


The next two ratings measure how much light a window lets into your home.

Visible Transmittance (VT)

This number between 0 and 1 measures what fraction of the spectrum of visible light the window lets through. The higher the fraction, the more light the window will allow. If you want to employ daylighting in your home, you’ll want a higher fraction. If you want to reduce interior glare, you may want a lower fraction.

Light-to-Solar Gain

This number is the ratio between the SHGC and the VT. “The higher the number, the more light transmitted without adding excessive amounts of heat.”

To learn more about energy efficient windows, contact Schaefer Siding and Exteriors today. We’ve been installing windows in Maryland since 1977. We’re both VSI certified and a CertainTeed 5-Star rated Contractor. Contact us today at 410-781-4028 or email us. You can also visit ourwebsite for more information.