R-Value of an Insulation Layer (R = L/k)

An R-value is thermal resistance per unit area: thickness divided by conductivity, with no area term at all. That is why an insulation label can carry one number for a product sold by the roll. 140 mm of mineral wool at k = 0.040 W/(m·K) gives 0.140/0.040 = RSI 3.5, and the same layer is R-19.9 in Canada or the United States — R and RSI are one quantity in two units, and because the imperial unit (h·ft²·°F/BTU) is the smaller one, R ≈ 5.678 × RSI. An "R-20 wall" and an "RSI-3.5 wall" are the same wall. Divide by 5.678 to go the other way; the calculator will do it for you, but the factor is worth memorising because half the arguments on a job site are really about this multiplier.

Rules of thumb by product, per inch of thickness: fibreglass and mineral wool batt R-3.1 to R-4.3, blown cellulose R-3.2 to R-3.8, expanded polystyrene R-3.6 to R-4.2, polyisocyanurate R-5 to R-6, and closed-cell spray foam about R-6. Softwood framing manages only R-1.25 per inch and concrete about R-0.08 — which is why an 8 in concrete wall is R-0.64 and needs everything it can get on the outside. The traps are all in k. It rises with temperature for fibrous insulation and falls for foams as the blowing agent diffuses out, so aged polyiso is derated; it collapses when the material gets wet, because water conducts about 25 times better than the trapped air it replaced; and compressing a batt into a shallower cavity costs you resistance in proportion, so an R-19 batt stuffed into a 3.5 in bay is delivering closer to R-13.