Guide
What Is Low-E Glass? (And What Argon Actually Does)
Low-E is a microscopically thin metallic coating on the glass. It reflects heat while still letting light through, so it keeps summer heat out and winter warmth in. Argon is a safe, invisible gas sealed between the panes that insulates better than plain air. Together they are most of what separates a modern window from old glass.
What Low-E actually is
"Low-E" is short for low emissivity — emissivity being a material's tendency to radiate heat. The coating is a metallic layer so thin you cannot see it, bonded to the glass surface. Visible light passes through it; a large share of the infrared that carries heat is reflected back the way it came.
That's the whole trick, and it works in both directions. In July it reflects the sun's heat back outside before it can load your air conditioning. In January it reflects the warmth of your room back into the room instead of letting it radiate out through the glass.
What's actually inside a modern window
The EPA breaks a window into three parts. It's worth knowing them, because a quote that only talks about one of them is only telling you a third of the story.
- The frame — wood, vinyl, fiberglass, composite, or aluminum.
- The hardware — locks, stops, balancers.
- The insulating glass unit (IGU) — the glass itself (two or three layers), the spacer holding the panes apart, the gas fill between them (air, argon, or krypton), and the Low-E coatings on the glass.
Argon lives in that gap. It is denser than air and a poorer conductor, so heat crosses it more reluctantly. It is inert, non-toxic, and already about one percent of the air you are breathing right now. You will never know it is there, which is rather the point.
U-Factor and SHGC: the only two numbers that matter
Every certified window carries two numbers. Learn them and you can compare any two windows in the country without a salesperson's help.
- U-Factor — how readily the window conducts heat. The EPA's own summary: as U-Factor improves, the window has more insulating power. Lower is better.
- SHGC (Solar Heat Gain Coefficient) — how much of the sun's heat the window lets through. As SHGC improves, more solar heat is blocked. Lower means more heat rejected.
ENERGY STAR certification is built on exactly these two, and the required thresholds change depending on where you live — which is where it gets interesting for Texas.
What ENERGY STAR requires where you live
These are the current Version 7 criteria for windows, which took effect in October 2023. North Texas sits in the South-Central zone — you can confirm your own county with the EPA's climate zone finder, linked below.
| Climate zone | U-Factor | SHGC |
|---|---|---|
| Northern | ≤ 0.22 | ≥ 0.17 |
| North-Central | ≤ 0.25 | ≤ 0.40 |
| South-Central (North Texas) | ≤ 0.28 | ≤ 0.23 |
| Southern | ≤ 0.32 | ≤ 0.23 |
Air leakage must also be ≤ 0.3 cfm/ft² in every zone. Version 7 tightened the South-Central requirements from Version 6, where the thresholds were U-Factor ≤ 0.30 and SHGC ≤ 0.25 — so a window that qualified a few years ago may not qualify today.
Why SHGC matters more than U-Factor in Texas
Look down the SHGC column again. In the North-Central zone the requirement is ≤ 0.40. In our South-Central zone it is ≤ 0.23 — very nearly twice as strict. Meanwhile the U-Factor requirements between those two zones differ by only 0.03.
The EPA is telling you something with that gap. Up north, the binding problem is holding heat in. Here, it is keeping the sun's heat out, and that is overwhelmingly a job for the Low-E coating rather than the gas fill. If a salesperson in Dallas is selling you hard on insulation value and cannot tell you the SHGC, they are optimizing for the wrong hemisphere.