R-value measures how hard it is for heat to get through a layer, so bigger is better — and it is the number American insulation is sold by. Materials compete on R per inch: multiply by installed thickness and you have the layer’s resistance. But a material does not own a single number; fiberglass spans 2.2 per inch blown loose in an attic to 4.3 in a high-density batt, purely on density, which is why the DOE-derived tables publish verified ranges — and quote loose-fill as settled values, the R you actually own a decade after installation.
The number comes in two dialects that differ by a factor of 5.678: a US R-value is in ft²·°F·h/BTU, a metric one in m²·K/W, so an American R-13 batt is R-2.29 metric and an unconverted figure is off by nearly a factor of six. US code requirements are imperial-R tables — under the 2021 IECC prescriptive path, attics need R-30 in zone 1, R-49 in zones 2–3, and R-60 in zones 4–8 — and converting a requirement into depth is division rounded up: an R-60 attic in 3.5-R/in cellulose takes ceil(60/3.5) = 18 inches.
Two honesty rules keep R-values from flattering a design. For foam boards, use aged rather than labeled figures: polyiso labels R-6.0–6.5 per inch as LTTR but NRCA recommends designing at 5.7 (5.0 in cold climates), and XPS labels 5.0 while its manufacturer warranty stands behind only 90% of that, about R-4.5 aged. And a wall of R-20 batts does not perform at R-20 — framing short-circuits the cavity (a wood stud runs about R-1.2 per inch), so whole-assembly performance is an area-weighted U-value calculation, not the batt label.