R-value
Also called: thermal resistance, R value, insulation rating
A measure of how well insulation resists heat flow. Higher is better. Fiberglass batts run about R-3 per inch, rigid foam R-4 to R-6, closed-cell spray foam R-6 to R-7. Codes and DOE guidance specify a target R-value by climate zone and location in the house.
R-value adds up: two R-15 layers make R-30. It only counts when the insulation is installed properly — compressed batts, gaps, and air moving through or around the insulation all cut the effective value, sometimes by half. That’s why air sealing matters as much as the rating on the package.
Targets for floors over unheated crawl spaces, per the U.S. Department of Energy: R-13 to R-19 in warm climates (zones 1–3), R-25 to R-30 in the rest of the country (zones 4–8). For crawl space walls: R-5 to R-10 in mild climates, R-10 to R-15 in cold ones.
Practical limit: the batt has to fit the cavity. 2×6 joists hold R-19; 2×8 hold R-25; 2×10 hold R-30. Stuffing a thicker batt into a shallower bay lowers the R-value rather than raising it.