When you are looking for windows for your home, how can you be sure that you are choosing the best option? By understanding these basic window rating measurements, you can make sure you are picking the best fit for your needs.

Window Rating Labels

The National Fenestration Rating Council, or NFRC, is a third-party non-profit organization that provides consumers with window, door, and skylight ratings. Windows that are certified by the NFRC have been independently tested and certified. 

The four main ratings that are included on every NFRC-certified window label are U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, and Air Leakage. Some window manufacturers may also include the optional Condensation Resistance rating on their labels.

ENERGY STAR qualifying windows have NFRC ratings that have met strict energy efficiency guidelines set by the U.S. Department of Energy. ENERGY STAR considers regional location when certifying windows because differences in climate affect what ratings are necessary to be energy efficient.


U-Factor is a measure of how well a window can keep heat from escaping. 

  • The range for U-Factor is between 0.20 and 1.20

  • The lower the U-Factor value, the less heat a window allows to escape.

Look for windows with a lower U-Factor to have a greater resistance to heat flow and better insulating value.

Solar Heat Gain Coefficient (SHGC)

The Solar Heat Gain Coefficient (SHGC) value measures how well a window resists unwanted heat gain from the sun.

  • The range for SHGC is between 0 and 1.

  • The lower the SHGC value, the better a window’s resistance is to heat gain from the sun.

Look for windows with lower SHGC values to reduce your cooling bill in the hot summer months.

Visible Transmittance (VT)

Visible Transmittance (VT) is the measurement of how well daylight is able to get through a window. This rating is especially important to consider if you want high levels of natural light to come into a room. If you are looking to save money on artificial lighting, pay attention to this rating.

  • The range for VT is between 0 and 1.

  • The higher the VT value, the more natural light a window lets in.

Look for windows with high VT values if you want to maximize the amount of natural light that comes into your home.

Air Leakage

Air Leakage measures how much air will enter a room through a window. This measurement represents the number of cubic feet of air passing through a square foot of window area per minute (cfm/sq. ft.).

  • The lower the Air Leakage value, the less air is able to enter a room through a window.

  • This value is assuming proper installation. Faulty installation could lead to damaged seals that could allow in more air. This is why you should always hire a trusted contractor to expertly install your new windows!

Condensation Resistance 

Condensation Resistance measurements are optional for manufacturers to include on NFRC labels. Condensation Resistance measures how susceptible to condensation buildup a window is. Condensation could lead to mold, discoloration, and warped wood. If it occurs between the layers of glass, condensation cannot be cleaned off and could block your view.

  • The range for Condensation Resistance is between 1 to 100.

  • The higher the value, the more resistant to condensation a window is.

  • Most ENERGY STAR qualifying windows have high Condensation Resistance ratings.

Now you know how to understand the basic window ratings that are included on NFRC window labels! Are you thinking about replacing the windows in your home? We can help guide you through the window replacement process! Start by requesting a free quote online today.