NFRC is a condensation rating that manufacturers can include on their labels. It shows testing results of the U-Factor, Visible Transmittance, Solar Heat Gain Coefficient, and Air Leakage. 

  • U-factor gives the rate of heat transfer through the window-from inside to outside when it is cold and from outside to inside when it is hot. The lower the U-factor, the better the insulating properties, resulting in greater energy efficiency. Range: 0.20-1.20. 
  • Visible Transmittance measures how well a product is designed to effectively light your home with daylight, which potentially saves you money on artificial lighting. The higher the number, the more natural light let it in. Range: 0-1. 
  • Solar heat gain coefficient (SHGC) is defined when sunlight hits a window and the solar heat is absorbed and subsequently released inward. The SHGC represents the fractional amount of the solar energy that ends up warming the house. The lower the SHGC, the less solar heat it transmits. Range: 0-1. 
  • Air Leakage measures how much air will enter a room through a product. The lower the number, the fewer drafts you’ll experience. Range: 0.3.  

Read more on https://www.nfrc.org