What is better, R13 or R30?

R30 is much better than R13 in terms of thermal resistance, but better depends on where you are using it because they are designed for different applications and cannot be interchanged. R-value measures resistance to heat flow, so higher means better insulation. R30 provides more than double the insulating power of R13. R30 is about 9.5 inches thick and is designed for attic floors, cathedral ceilings, and floors over unconditioned spaces like 2x10 joists. R13 is 3.5 inches thick and is designed to fit in a standard 2x4 wall cavity. You cannot properly use R30 in a 2x4 wall because if you compress a 9.5-inch batt into a 3.5-inch cavity, it loses most of its R-value and will perform at only about R12 to R14 while causing drywall bulges. Conversely, using R13 in an attic would be far below code, as most states require R38 to R60 in attics. So if you are comparing them for the same location, R30 is always better thermally. For example, in an attic, R30 saves about 40 percent more energy than R13. In a wall, if you have 2x4 studs, R13 is the correct product, but R15 high-density batt is better than R13 for the same cavity. If you have 2x6 walls, neither R13 nor R30 is ideal, you need R19 to R21. Always match insulation thickness to cavity depth.