What R-value means — the same R-30 takes very different thickness by material
R-value = R-per-inch × thickness. To hit R-30, closed-cell foam needs ~4.6″ but a fiberglass batt needs ~10.3″ — same R, very different depth. Compare materials by R-value, not by inches.
What this diagram shows
Five insulation materials drawn as columns on a common baseline, each at the real depth it takes to reach the same R-30. Closed-cell spray foam at R-6.5 per inch needs only about 4.6 inches; XPS rigid board at R-5.0 per inch needs 6 inches; open-cell spray foam at R-3.6 per inch needs 8.3 inches; blown cellulose at about R-3.3 per inch needs 9.1 inches; and a fiberglass batt at about R-2.9 per inch needs 10.3 inches. R-value equals R-per-inch times thickness, so a higher R-per-inch material reaches the same R-value in less depth. To compare materials honestly, compare the R-value, not the inches.
Insulation Calculator
Free insulation calculator for fiberglass batts, blown-in cellulose, spray foam & rigid board. Bags by square footage, R-value by climate zone, code check.
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