U-factor measures the rate at which a window, door, or skylight conducts non-solar heat. It is the number that tells you how quickly your heating dollars leave the building through the glass.
Lower is better. A U-factor of 0.20 loses heat more slowly than 0.30.
The units, because people search for this specifically
In the United States, U-factor is expressed in BTU per hour, per square foot, per degree Fahrenheit, written as BTU/(hr sq ft F). Internationally it appears in watts per square meter per kelvin, W/(m2 K). To convert, multiply the US figure by roughly 5.68.
U-factor is the mathematical inverse of R-value, the insulation number you already know from attic work. U = 1 divided by R. A window with a U-factor of 0.25 has an R-value of 4. That comparison is worth holding onto, because it explains something uncomfortable: a very good window performs about as well as four inches of nothing much. Walls are insulated to R-13 or better. Windows are the hole in your thermal envelope, and the best available product only narrows the gap.
What counts as a good number
| Window type | Typical U-factor |
|---|---|
| Single pane, clear | 0.90 to 1.30 |
| Double pane, clear | 0.45 to 0.55 |
| Double pane, low-E and argon | 0.25 to 0.32 |
| Triple pane, low-E and argon | 0.15 to 0.22 |
ENERGY STAR Version 7.0 sets the current thresholds. In the Northern zone, windows must hit a U-factor of 0.22 or lower, with a minimum SHGC of 0.17. In the South-Central zone, the requirement tightened to 0.28 or lower, with SHGC at 0.23 or lower.
Where buyers get hurt
The U-factor on the brochure is not always the U-factor you are buying.
Center-of-glass versus whole-window. Manufacturers can quote the performance of the glass alone, which ignores the frame and the edge seal where most heat actually escapes. The NFRC label reports whole-window performance. If a salesperson quotes you a number that is not on an NFRC label, you are being shown the flattering version.
The wrong number for your climate. A superb Northern-zone window installed in Phoenix can make a house worse, because in a cooling climate solar heat gain matters more than conductive loss. Buying the lowest U-factor available regardless of where you live is a common and expensive mistake.
Installation erases the rating. A 0.20 window installed with gaps, missing insulation around the frame, or a failed air seal performs like a far worse window. The label rates the product. It does not rate the crew, and the crew is the variable you actually chose when you signed.
The honest version: U-factor is one of three numbers that matter, alongside SHGC and the quality of the installation. Two of those three are printed on a label. The third one decides whether the label meant anything.