Total R-Value
R-19.2
R-40.8 short of R-60
Recommended
R-60
zone 4 attic / ceiling
Depth Needed
18.8"
12.8" more
How we calculated this
= R-19.2
Insulation R-value only; framing, sheathing, and air films add or subtract a little. Compressed batts and settled blown-in deliver less than rated.
Reference
Recommended R-Value by Climate Zone
Simplified 2021 IECC residential minimums. Local codes vary; check yours before a permit job.
| Climate Zone | Attic | Wall | Floor |
|---|---|---|---|
| Zone 1 · S. FL, HI | R-30 | R-13 | R-13 |
| Zone 2 · Gulf Coast, S. TX, AZ desert | R-49 | R-13 | R-13 |
| Zone 3 · S. CA, GA, NC coast | R-49 | R-20 | R-19 |
| Zone 4 · Mid-Atlantic, KY, OR coast | R-60 | R-25 | R-30 |
| Zone 5 · OH, IL, NV high desert | R-60 | R-25 | R-30 |
| Zone 6 · MN, WI, New England | R-60 | R-25 | R-30 |
| Zone 7 · N. MN, ND, mountain west | R-60 | R-25 | R-30 |
| Zone 8 · Interior AK | R-60 | R-25 | R-30 |
Learn
Understanding R-Value
What R-value measures
R-value is resistance to heat flow: how hard it is for heat to get through a material. The higher the number, the slower the heat moves, in either direction, which is why insulation works in both winter and summer. It's additive across layers and proportional to thickness within a material, which makes the math unusually honest: double the depth, double the R.
R-value per inch by material
The spread is wide. Blown fiberglass sits at R-2.5 per inch, fiberglass batt at 3.2, cellulose at 3.5, and open-cell spray foam at 3.7. The foams pull ahead: EPS at 4.0, XPS at 5.0, polyiso at 6.0, and closed-cell spray foam at 6.5. Per dollar the ranking roughly inverts, with blown cellulose and fiberglass cheapest per unit of R, which is why attics get loose fill and tight wall cavities get foam.
Climate zones set the target
The DOE divides the country into climate zones 1 (south Florida) through 8 (interior Alaska), and the model energy code (IECC) sets recommended R-values per zone: more R where winters are harder. Attics carry the highest targets, R-30 to R-60, because heat rises and the attic is the easiest place to install deep insulation. Walls and floors target less, partly because cavity depth physically limits what fits.
Where the rated R goes missing
The number on the bag assumes a perfect installation. Compressed batts, gaps around boxes and wiring, settled blown-in, and thermal bridging through studs all shave real-world performance. Wood studs are only about R-1.2 per inch, so a 2×6 wall that's 25% framing performs well below its cavity R. This is why continuous exterior foam, which covers the studs, is required by code in colder zones.
Air sealing comes first
Insulation slows conductive heat loss, but it does little against air leaking through gaps, and a typical older home loses a third or more of its heat to air leakage. Sealing the attic plane, rim joists, and penetrations before adding R is the standard weatherization order, and it's also why the blower-door test exists. R-value answers "how good is the blanket"; air sealing answers "is the window open under it."
FAQ
Common Questions About R-Value
How do you calculate R-value?
Multiply the material's R-value per inch by the installed thickness. Fiberglass batt runs about R-3.2 per inch, so a 5.5" 2×6 wall cavity holds R-17.6. Layers simply add: R-19 batt plus R-5 rigid foam sheathing is R-24. This calculator does the per-material math and compares it to the recommendation for your zone.
What R-value do I need in my attic?
By the 2021 IECC: R-30 in zone 1, R-49 in zones 2–3, and R-60 in zones 4–8. In blown cellulose (R-3.5/inch) that's roughly 9" to 17" of depth. The attic is the cheapest place in the house to add insulation, so exceeding the minimum usually makes sense if the depth is there.
What insulation has the highest R-value per inch?
Closed-cell spray foam (R-6.5/inch) and polyiso rigid foam (R-6.0) lead, followed by XPS (R-5.0) and EPS (R-4.0). The fibrous materials cluster lower: open-cell foam R-3.7, cellulose R-3.5, mineral wool R-3.3, fiberglass batt R-3.2, and blown fiberglass R-2.5. High R per inch matters most where depth is fixed, like a 2×4 wall or a cathedral ceiling; in an open attic, cheap material and more depth wins.
Is a higher R-value always better?
Returns diminish quickly. Heat flow through an assembly is proportional to 1/R, so going from R-11 to R-19 cuts the loss by about 40%, while R-19 to R-30 only saves another 20% or so. Once you're near the recommended level, the bigger wins are usually air sealing and fixing gaps; a 5% void in a wall can cost more heat than several added R points would save.
Does compressing insulation reduce its R-value?
Yes. Compressing a batt raises its R per inch a little but lowers its total R, and the total is what matters. An R-19 batt (designed for 6.25") squeezed into a 3.5" 2×4 cavity delivers about R-13. The same applies to blown-in insulation that settles, and to batts stuffed around wires and pipes instead of split around them. Size the product to the cavity.
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