Every window quote comes with a spec sheet full of numbers and acronyms. Two of them matter more than the rest: U-Factor and SHGC. Here’s what they mean, and how to read them in a real quote.
U-Factor: How Much Heat Escapes
U-Factor measures how well a window insulates; specifically how much heat passes through it. The lower the number, the better the insulation, the opposite of R-value logic. It’s rated per unit, tested with a specific glazing package, and it changes depending on grid options and glass thickness.
To put real numbers next to it, here’s how Value’s series compare on a standard slider, no grids:
U-Factor and SHGC by Series (Slider, No Grids)
| Series | Glazing | U-Factor | SHGC |
|---|---|---|---|
| Imperial | Dual-pane LoE3 + Argon | 0.32 | 0.29 |
| Galaxy | Dual-pane LoE3 + Argon | 0.21 | 0.19 |
| GS (Triple Pane) | Triple-pane LoE3 + Argon | 0.20 | 0.35 |
| GS DuraPane | Laminated dual-pane + Argon | 0.28 | 0.47 |
| EUROtek | Fixed window | 0.15 | 0.17 |
| Aluminum | Triple-pane LoE3 + Duralite | 0.32 | 0.21 |
GS’s DuraPane option trades a bit of U-Factor for a big jump in sound rating (more on that below). On oversized picture windows, Value’s thermally broken aluminum line reaches an even lower 0.24 U-Factor with triple glaze, a strong number for an aluminum frame.
Rule of thumb: in most U.S. climates, look for U-Factor at or below 0.30 for real energy savings. If you’re chasing California Title 24 compliance, triple-pane plus LoE3 plus a warm-edge spacer (Duralite) is usually the combination that gets you there. On aluminum, that combination is explicitly what makes it Title 24 compliant.
SHGC: How Much Solar Heat Gets In
Solar Heat Gain Coefficient measures the fraction of solar heat that passes through the glass, on a scale from 0 to 1. Lower SHGC blocks more solar heat, which is good for hot climates and south- or west-facing glass. Higher SHGC lets more free solar heat in, useful in colder climates where passive heat gain lowers heating bills.
The SHGC column in the table above shows how this plays out across Value’s lineup: GS’s DuraPane option runs highest at 0.47 (a laminated build tuned more for sound and impact resistance than solar control), while Galaxy and Aluminum T-5000 sit lowest at 0.19 and 0.21. There’s no universal “best” SHGC. It depends entirely on your climate and which elevation the window faces.
Why the Two Numbers Work Together
A window can have a great U-Factor and a poor SHGC for your climate, or vice versa. You need both to make a real comparison. A quick gut-check:
- Hot, sunny climate (most of California): prioritize low SHGC (0.20–0.25) and a solid U-Factor (0.30 or lower).
- Mixed or cold climate: a slightly higher SHGC can work in your favor on south-facing glass, as long as U-Factor stays low.
Other Numbers You’ll See Nearby
- VT (Visible Transmittance): how much visible light gets through. Unrelated to heat, it matters for daylighting.
- STC (Sound Transmission Class): higher is quieter. GS’s DuraPane laminated glass reaches STC 33–34, well above standard dual-pane options.
- CR (Condensation Resistance): higher resists condensation better in humid or cold-swing climates.
Don’t compare windows on price alone. Ask for U-Factor and SHGC on the exact glazing package you’re quoted, not just the “as tested” number on a spec sheet. A $200 difference in a quote often comes down to a triple-pane upgrade or a different spacer, and that’s exactly what moves these two numbers.
