What R-Value Do I Need?

Zones 1–2 (Gulf Coast, south Florida, south Texas): attic R-30 to R-49, walls R-13 to R-15, floors R-13.

Zone 3 (Southeast, southern California): attic R-38 to R-49, walls R-13 to R-20, floors R-19.

Zone 4 (mid-Atlantic, mid-South): attic R-49, walls R-20 or R-13 plus R-5 continuous, floors R-19 to R-25.

Zones 5–6 (Midwest, Northeast, mountain West): attic R-49 to R-60, walls R-20 to R-21 plus continuous foam, floors R-30, basement walls R-15 to R-19.

Zones 7–8 (northern Minnesota, Maine, Alaska): attic R-60, walls R-21 plus R-10 continuous, floors R-38.

Divide your target by the material's R per inch. R-49 in cellulose (R-3.6/in) is about 14 inches; in blown fiberglass (R-2.5/in) about 19 inches; in closed-cell spray foam (R-6.5/in) about 7.5 inches.

Going from R-0 to R-13 eliminates about 92% of conductive loss through that surface. R-13 to R-38 picks up another 5%. Beyond R-60 the change is barely measurable. Spend the first dollars on the worst surfaces, not the last inch of an already-good attic.

Wood framing is about R-1.25 per inch, so a wall of R-21 batts between studs performs closer to R-16 overall. Continuous exterior foam is how you close that gap — an inch of polyiso over the sheathing does more than upgrading the cavity batt.