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Making Sense of Performance Ratings for Replacement Windows


Old windows can be a drain on your wallet.  Leaky windows allow heat to escape, causing your HVAC system to run more and your utility bills to increase.  By replacing old windows, you'll notice an immediate difference in both the comfort of your home as well as lower utility bills.  

When you're shopping for replacement windows, you'll soon realize there's a learning curve. Window construction materials, styles, glazing, energy ratings - it can be overwhelming for some.  Educating yourself on the key features and terminology will go a long way in making a wise choice for your replacement windows.

First and foremost, if you're investing in a window replacement project, it makes sense to choose ENERGY STAR® certified windows.  For a window to be considered ENERGY STAR certified, it must go through testing performed by the National Fenestration Rating Council (NFRC).  This testing shows how well the windows perform in various conditions and climates. All the test ratings can be found on the ENERGY STAR label on the qualified window.

The NFRC performance ratings indicate how well the windows handle heat transmittance. Windows can have heat gain and loss through a number of ways.  Besides having a leaky window, heat can also enter your home through the glass itself, knows as conduction.  Key NFRC ratings pertaining to heat gain and loss that you need to understand include:

  • Solar heat gain coefficient (SHGC) - This is the amount of solar radiation that passes through your window.  The lower the SHGC, the less solar heat it transmits, making it more effective in the summer by blocking solar heat gain.  A higher SHGC is more effective in the winter at collecting solar heat.
  • U-factor - While the SHGC measures solar heat flow, the U-factor measures non-solar heat flow.  U-factor is mainly affected by the glass, but the window frame and spacer material also affect non-solar heat flow.  A lower U-factor means the window is more energy efficient.
  • Air Leakage - As you can guess, this term refers to the amount air movement around the window.  A lower air leakage rating indicates the window is more air-tight.
  • Visible transmittance (VT) - This number refers to the amount of sunlight that is transmitted through the window glass.  A lower VT means less visible light enters through the window, which may be desirable if the window is south facing and receives a lot of direct sunlight.

By choosing an ENERGY STAR window, you know you're making an energy efficient choice.  But by understanding the NFRC ratings on the ENERGY STAR label, you'll be able to compare various certified windows to make the best choice for your home.