One label, four numbers, one decision
Every window certified through the National Fenestration Rating Council carries a label reporting a small set of standardized performance numbers, typically shown in the lower corner of the glass or printed on an accompanying certificate. Reading it correctly matters more than any brand name on the frame.
U-Factor: how much heat moves through the whole assembly
U-factor measures the rate of heat transfer through the entire window assembly, expressed in Btu per hour per square foot per degree of temperature difference between inside and outside.[1]
Lower is always better for U-factor, regardless of climate: it means less heat escapes in winter and less heat intrudes in summer. Single-pane clear glass sits up near 1.09; a strong modern window can run well under 0.30.
SHGC: how much solar heat gets in
Solar Heat Gain Coefficient is the fraction of solar heat that passes through the window into the home, on a scale from 0 to 1.[1]
Unlike U-factor, a lower SHGC isn't automatically better. In a hot climate, a low SHGC keeps unwanted solar heat out. In a cold climate with good south-facing exposure, a higher SHGC can be an asset, since it lets in warmth you'd otherwise have to generate with a furnace. That's the entire reason ENERGY STAR sets SHGC as a ceiling in three of its four climate zones but a floor in the fourth.
Visible Transmittance: how much light gets through
Visible Transmittance reports how much of the visible light spectrum passes through the glass, on a 0-to-1 scale where a higher number means more natural daylight in the room. A very low-SHGC coating can sometimes pull VT down with it, which is worth checking if you're prioritizing daylight in a specific room.[2]
Air Leakage: how much air sneaks through the frame
Air Leakage measures the volume of air passing through the assembly, in cubic feet per minute per square foot of area, due to pressure or temperature differences between inside and out.[1]
For a window to qualify under the current ENERGY STAR specification, it has to hit an Air Leakage rating of 0.3 cfm per square foot or better, one of the few NFRC ratings with a single national cutoff rather than a zone-by-zone target.[1]
Condensation Resistance: the rating most labels skip
Some labels also carry a Condensation Resistance rating, a relative 1-to-100 scale where a higher number indicates better resistance to interior condensation forming on the glass. It's not required for every certified product, so its absence from a label isn't itself a red flag.
Reading a label for your specific zone
Once you know your ENERGY STAR zone from the companion guide on this site, hold the U-factor and SHGC on any proposed product's label directly against that zone's numbers. A product that beats the requirement is a genuine upgrade; a product that merely meets it is meeting a floor, not offering you anything extra.