Single-pane aluminum frame (1960s): U-1.2 to U-1.4 (R-0.7 to R-0.8) - Double-pane aluminum frame, no thermal break (1980s): U-0.6 to U-0.8 (R-1.25 to R-1.67) - Double-pane vinyl, standard low-e, argon: U-0.28 to U-0.35 (R-2.9 to R-3.6) - Triple-pane fiberglass, low-e, krypton: U-0.17 to U-0.22 (R-4.