A good R-value is one that meets or exceeds energy code for your location and provides a strong return on investment in comfort and energy savings without excessive cost or diminishing returns. As a rule of thumb, a good R-value for an attic is R-49, which is the sweet spot for most of the United States balancing cost and performance, requiring about 14 inches of cellulose or 16 inches of fiberglass. A good R-value for a 2x4 wall is R-13 to R-15 cavity plus R-5 continuous exterior for a total of R-20, and for a 2x6 wall R-20 to R-23 cavity plus R-5 to R-10 continuous for a total of R-25 to R-33. A good R-value for floors over unconditioned space is R-25 to R-30, and for basement walls R-10 to R-15 continuous. The concept of good also includes quality of installation, an R-19 batt installed with gaps and compression may perform as R-13, while an R-13 batt installed perfectly with complete air sealing may outperform it. So a good R-value is not just a number, it is the installed performance that includes proper air sealing, no thermal bridging, and moisture control, achieving comfort, low bills, and durability.
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