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.