The highest R-value you can achieve in a standard 3.5 inch deep 2x4 wall cavity alone is about R-22 to R-23 using closed-cell spray foam at R-6.5 to R-7 per inch filling the entire cavity. With fiberglass, the highest you can get in a 2x4 cavity is R-15 using a high-density batt specifically designed for 3.5 inches, while standard batts give R-13. With cellulose dense-packed at 3.5 pounds per cubic foot you get about R-12 to R-13. However, if you consider the whole wall assembly including continuous exterior insulation, you can go much higher than the cavity limit. By adding 2 inches of polyiso R-13 or 2 inches of closed-cell foam board outside the sheathing over a 2x4 wall with R-15 batts, you can achieve a total nominal R-28, and with 3 inches of exterior mineral wool or polyiso you can reach R-30 to R-35. With vacuum insulated panels on the exterior you could theoretically exceed R-50, but that is not common. So cavity-only max is about R-22 to R-23 with spray foam, and with exterior foam the practical high-performance max is R-30 to R-40 for a 2x4 based wall.
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