Do I need R13 or R19 insulation?

Whether you need R13 or R19 insulation depends entirely on where you are installing it and the depth of your framing. R13 batts are 3.5 inches thick and are designed for 2x4 walls, which are standard in many older homes and interior walls. R19 batts are 6.25 inches thick and are designed for 2x6 walls or for floors and ceilings. You should not use them interchangeably. If you have 2x4 exterior walls, you need R13 or the upgraded R15 high-density batt. If you have 2x6 exterior walls, which are common in new construction and colder climates, you need R19 or ideally R20 to R21. For floors over an unheated crawl space or garage, R19 is the minimum, but R25 to R30 is recommended. For attics, neither R13 nor R19 is enough by itself. Current energy codes require R38 to R60 in attics, so you would need multiple layers. The other factor is your climate zone. In IECC climate zones 1 to 3 in the southern US, R13 in 2x4 walls meets code. In zones 4 to 8 in the northern US, code often requires R20 in walls, which means R13 alone will not pass unless you add continuous exterior foam insulation. Check your local building code and the Department of Energy's insulation recommendations for your zip code. When in doubt, match the batt to your stud depth and aim for the highest R-value that fits without compression.