Every window sold in the US carries an NFRC label, and it is the only document in the whole transaction that is independently certified. The brochure is marketing; the label is a test result.
The two everyone reads
U-factor — heat flow through the whole assembly. Lower is better, typically 0.15 to 1.20.
SHGC — fraction of solar energy that gets through. Lower is better in a cooling climate, higher on south glass in a heating one.
The three that decide how the window feels
Visible transmittance (VT) — the fraction of daylight that comes through, 0 to 1. A clear double-pane is around 0.60 to 0.70. Below about 0.40 a room noticeably darkens, and the lights go on earlier in the day. VT is where a cheap approach to low SHGC shows up: darkening the glass drops both together, while a good spectrally selective coating drops SHGC and leaves VT high.
Air leakage (AL) — cubic feet per minute per square foot of window, measured under a standard pressure. The NFRC maximum for a certified window is 0.3, and good units come in at 0.1 or below. This is the number that explains draughts, and it is routinely the difference between a window that feels expensive and one that does not.
Condensation resistance (CR) — a 1 to 100 scale, higher is better, describing how well the window resists condensation on the interior surface. It is optional and not every label carries it. Where it does appear, a number above about 55 is worth having in a cold climate, and it correlates with the spacer and the frame rather than the glass.
Air leakage is the one to look at
Conduction through the glass is a steady, invisible loss. Air moving through a gap is what you feel as a cold draught at ankle height, and it is what makes a room feel unheatable regardless of the thermostat.
Between two windows with the same U-factor, one at AL 0.30 and one at AL 0.05, the second will feel dramatically better and no efficiency label will explain why.
Operating type matters here more than brand. Roughly, from tightest to leakiest: fixed, casement and awning, single and double hung, horizontal slider. Casements compress against the frame when they latch; sliders run on a track and cannot seal the same way.
The label is per configuration, not per product
This is the most common misreading. A quote may cite one U-factor for the whole job while the house takes picture windows, casements and sliders from the same line — three different labels, three different numbers.
Grids between the glass change the numbers. Triple glazing changes them. Tempered glass changes them slightly. A different low-E package changes them a lot.
Ask for the NFRC-certified numbers for each line item on the order. A supplier who cannot produce them is quoting from a brochure.
What the label does not tell you
Three things that matter and are not on the sticker:
Installation quality. A perfectly rated window installed without flashing or air sealing performs worse than a mediocre window installed properly. Air leakage is tested on the unit, not the opening it goes in.
Durability. Nothing on the label speaks to how the seal, the hardware or the finish will look in fifteen years. Warranty terms are a rough proxy and a poor one.
Sound. Acoustic performance (STC or OITC) is a separate rating and rarely quoted. It correlates weakly with U-factor — laminated glass or unequal pane thicknesses do far more for noise than a third pane does.
The one-minute version
Read U-factor and SHGC against your climate zone. Then read air leakage, and treat anything above 0.20 as a warning. Then check that the numbers you were quoted match the line items you are actually buying.