IGU spec choices that actually move the U-value needle
Not all thermal specs are created equal. Here's where the real gains hide in an insulated glass unit.
A center-of-glass U-value on the cut sheet is only part of the story. The whole-window U-value — the number your energy model actually uses — depends on the spacer, the frame, and the installation as much as the glass.
Warm-edge spacers (typically a stainless-steel or polymer hybrid) reduce edge conduction by 30–50% compared to the aluminum spacers still shipped as standard by many fabricators. On a large expanse of glass this shows up as a full 0.05 U-value improvement — meaningful in code compliance.
Argon fill matters, but only if it stays in the unit. Look for fabricators that guarantee ≥90% argon retention at time of manufacture and use dual-seal construction (PIB primary, silicone secondary) to hold that fill for 20+ years.
Triple glazing is a code driver in high-altitude and cold-climate work, but the weight penalty (roughly 50% more than a comparable IGU) reshapes the framing, hardware, and structural load path. It's a systems decision, not a glass decision.