Double-pane low-E insulated glass units (IGUs) are the standard for energy-code-compliant windows in most US climate zones. Construction: two panes of glass (typically 3mm-4mm each) separated by a spacer (warm-edge spacer preferred), cavity filled with argon gas (krypton for premium), low-emissivity coating on surface 2 or 3. Performance: U-factor 0.25-0.30 (lower is better). Solar Heat Gain Coefficient 0.25-0.40 (lower blocks more solar heat). Visible Transmittance 0.40-0.70. IECC 2024 maximum U-factor by zone: Zone 1-3: 0.40, Zone 4: 0.32, Zone 5-8: 0.30. Triple-pane required for passive house and some Zone 6+ applications. Frame material matters: vinyl U-0.28 typical, aluminum U-0.45 (thermal break required), wood U-0.30, fiberglass U-0.27. NFRC label required — do not rely on manufacturer claims without NFRC certification.