How this insulation calculator works
R-value measures resistance to heat flow, and it stacks linearly: thickness times the material's R per inch. Fiberglass batts run about R-3.2 per inch, blown cellulose R-3.5, mineral wool R-4.2, EPS foam board R-4, XPS R-5, and closed-cell spray foam R-6 to R-7. The metric equivalent, RSI, is the R-value times 0.176. The buy figure adds 5% for trimming around framing.
Common targets by location
US recommendations vary by climate zone, but common targets are R-38 to R-60 in attics, R-13 to R-21 in walls (limited by stud depth), and R-25 to R-30 in floors over unheated spaces. Attics are the best value per dollar — heat rises, access is easy, and there is room for depth.
Thickness only counts uncompressed
A batt squeezed into a shallower cavity loses R-value — R-19 fiberglass compressed into a 2×4 wall performs around R-13. Match the batt to the cavity depth, cut around wiring and boxes rather than stuffing behind them, and air-seal gaps first: insulation slows heat, but only air sealing stops drafts.