From Wikipedia … The National Fenestration Rating Council (NFRC) is a United States 501(c)3 non-profit organization which sponsors an energy efficiency certification and labeling program for windows, doors, and skylights.[1]
NFRC labels provide performance ratings for such products in five categories: U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, Air Leakage, and Condensation Resistance.[1] This allows architects, builders, code officials, contractors, home owners, and specifiers to compare the energy efficiency among products, and determine whether a product meets code.
Here’s the video that NFRC uses to explain the window label.
From the Home Building / Remodeling web site:
On window labels, there are 5 basic pieces of information that are very important:
- U-factor measures how well a window prevents heat transfer. The lower the U-factor, the better a window’s resistance to heat flow. It also means it has a better insulating value. A window with a U-factor of 0.25 has an R-value of 4 (1/.25=4).
- Solar Heat Gain Coefficient (SHGC) determines how well a window blocks heat from sunlight. SHGC is measured from 0 to 1 and it’s the fraction of incident solar radiation through a window. The lower the number, the less solar heat transmits into the home.
- Visible Transmittance (VT) is how much light comes through a window or skylight. It’s also measured between 0 and 1. The lower the number means the lesser amount of visible light shines through. If a window has a VT of 0.85, that means that 85% of the light will transmit through the window.
- Air Leakage (AL) measures air infiltration through cracks in the frame, sash, and the window as a whole. The rating is shown as equivalent cubic feet of air that passes through a square foot of window. The lower the number, the lower amount of air that passes through.
- Condensation Resistance (CR) is the ability of the window to resist the formation of condensation on the inside of the window during colder weather. CR is measured between 0 and 100 with the higher rating indicating that it’s better at resisting condensation forming on the interior of the window.