Window ratings explained: U-factor, SHGC, and low-E
Every new window carries an NFRC label crowded with numbers, and the right ones for a hot climate are the wrong ones for a cold one. Here is what U-factor, SHGC, and the rest actually mean, and how to read the label for where you live instead of memorizing it.
Key takeaways
- U-factor measures heat loss; lower is better, and it matters most in heating climates. Typical range is about 0.20 to 0.40.
- SHGC (Solar Heat Gain Coefficient) measures how much solar heat the window lets in; lower keeps you cooler, higher helps in cold climates.
- Low-E coatings are microscopically thin metal layers that cut heat transfer; the coating is tuned for northern vs. southern climates.
- Cold climates want a low U-factor; hot climates want a low SHGC; mixed climates want a balance of both.
- The simplest move: buy windows that carry the ENERGY STAR label for your climate zone.
The two numbers that matter most
Two more appear on the label: Visible Transmittance (how much daylight comes through) and Air Leakage (lower is tighter). They matter less for energy than U-factor and SHGC, but a low air-leakage rating is a sign of a well-built window.
- U-factor: how fast the window loses heat. Lower means better insulation. It usually runs from about 0.20 (excellent) to 0.40, and it is the number to watch in a heating climate.
- SHGC: the Solar Heat Gain Coefficient, from 0 to 1, is how much of the sun's heat passes through. A low SHGC keeps a hot-climate home cooler; a higher SHGC adds welcome warmth in a cold one.
What low-E glass does
Low-emissivity (low-E) glass has a microscopically thin metallic coating that reflects heat while letting light through. The coating is tuned by climate: a northern low-E keeps furnace heat inside and lets some solar warmth in, while a southern low-E blocks more solar heat to ease the air conditioning load. Paired with an inert gas fill (argon or krypton) between the panes and a warm-edge spacer, low-E is what separates a modern window from an old clear-glass one.
Read the label for your climate
ENERGY STAR divides the country into four climate zones and sets a U-factor and SHGC target for each. The pattern is intuitive: colder zones demand a lower U-factor, hotter zones a lower SHGC.
| Climate zone | Max U-factor | SHGC |
|---|---|---|
| Northern (heating) | 0.22 | Any, or higher to capture solar heat |
| North-Central | 0.25 | 0.40 or lower |
| South-Central | 0.28 | 0.23 or lower |
| Southern (cooling) | 0.32 | 0.23 or lower |
The shortcut
You do not have to memorize any of this. Look for the ENERGY STAR mark for your climate zone on the NFRC label, and the window already meets the right U-factor and SHGC targets for where you live. Use the individual numbers only when you are comparing two qualifying windows or fine-tuning by room, for instance choosing a lower SHGC on the sun-blasted west side of the house.
Frequently asked questions
What is a good U-factor for windows?
What is SHGC, and should it be high or low?
What does low-E glass do?
See the numbers for your town
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Cost figures in this guide are modeled national ranges for general planning, not quotes. Local pricing varies, always get an on-site assessment from a licensed pro before you commit. Evergreen guide