Windows come with energy performance ratings that help you compare how well different windows save energy.
Window energy performance ratings are issued by the National Fenestration Rating Council (NFRC), a non-profit organization with the mission of creating objective standards for measuring the energy efficiency of windows, doors, and skylights.
NFRC ratings give you guidelines for understanding how a window, door, or skylight performs on conserving energy and allowing sunlight and UV radiation into the building.
You can find the NFRC rating on all Energy Star® products, informing you of the U-factor and solar heat gain coefficient for that product.
Here is what those terms mean.
U-factor Energy Ratings for Windows
The U-factor rating on a window, door, or skylight tells you how fast the product conducts heat through its surface. The NFRC’s U-factor rating gives you this information for the entire unit, including the frame, spacers, and glass surfaces.
SHGC Ratings for Windows
The solar heat gain coefficient (SHGC) tells you what percentage of the sun’s rays can penetrate the surface of a window and heat the space inside.
A lower number indicates better insulation against solar heat transfer from outside to inside the building, an advantage for cooling in the summer.
A higher SHGC number means the inside of the building will retain heat more efficiently in winter.
Other Energy Ratings on Windows
Many brands of windows, skylights, and doors also provide a rating for air leakage. This rating measures how much air can pass around the frame of the unit.
Air leakage is shown as cubic feet of air per square feet of the entire surface of the unit (cfm/ft2.) Lower ratings indicate that less air can pass through the surface.