Two Numbers, Two Different Jobs
Every window sold in the U.S. carries a label from the National Fenestration Rating Council (NFRC), an independent, non-profit rating body separate from the manufacturer. That label reports several ratings, but two matter most for climate performance: U-factor and Solar Heat Gain Coefficient (SHGC). They sound similar and both get lumped under "energy efficient," but they measure two physically different things.
U-Factor: How Fast Heat Moves Through the Window
U-factor measures the rate of non-solar heat transfer, meaning heat conducting through the glass and frame because it's colder (or hotter) on one side than the other. It's expressed in Btu per hour per square foot per degree Fahrenheit of temperature difference, and it typically ranges from around 0.20 (very well insulated) up to 1.2 or higher (a single pane of clear glass in an old aluminum frame). Lower is always better here. There's no climate where a higher U-factor helps.
SHGC: How Much Solar Heat Gets Through
SHGC measures the fraction of solar radiation that passes through the glass and turns into heat inside the room, on a scale of 0 to 1. A higher SHGC lets in more free solar heat. That's useful on a south-facing window in winter and unwanted on a west-facing window in July. Unlike U-factor, SHGC isn't "lower is better" everywhere. It depends on orientation and season, which is why it trips people up.
| Rating | What it measures | Lower number means |
|---|---|---|
| U-Factor | Heat conducted through glass & frame | Better insulation, always preferred |
| SHGC | Solar heat passed through the glass | Less solar heat gain: good in summer sun, a tradeoff in winter |
Why Albuquerque Needs Both Numbers, Not Just One
A climate that's purely cold cares almost exclusively about U-factor, because the goal is keeping heat in. A climate that's purely hot cares mostly about SHGC and keeping the sun's heat out. Albuquerque's high-desert climate does both. Cold, clear nights call for a low U-factor, and intense high-altitude summer sun (see our companion article on why that sun hits harder here) calls for a moderate-to-low SHGC on sun-exposed walls. ENERGY STAR's own regional criteria reflect this by requiring stricter numbers for both ratings in this part of the country than they would for a milder climate zone.
Reading the Label in Practice
- Look at both numbers together, not one in isolation. A window with an excellent U-factor but a high SHGC can still overheat a west-facing room.
- Match the SHGC to the wall's orientation where possible. A south-facing window can sometimes benefit from a slightly higher SHGC than a west-facing one, since it collects useful winter sun at a lower, more manageable angle.
- The NFRC label is standardized across manufacturers, so it's one of the few specs on a window quote you can compare directly, brand to brand.