What Is the Recommended Attic R-Value by Climate Zone?
The short answer
The recommended attic R-value is R-30 in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 7. Those are the ceiling insulation levels in the 2021 IECC (the US model energy code, Table R402.1.3), and Energy Star’s attic guidance points to the same targets - hotter zones need less, colder zones need more.
Zone 8 (arctic Alaska) is also R-60, so from zone 4 north the target simply stays at R-60.
Attic R-value by climate zone
| Climate zone | Where that is (typical) | Recommended attic R |
|---|---|---|
| 1 | South Florida tip, Hawaii | R-30 |
| 2 | Most of Florida, the Gulf Coast, southern Texas, the Phoenix desert | R-49 |
| 3 | The Deep South, Dallas, Las Vegas, most of coastal California | R-49 |
| 4 | Mid-Atlantic, much of Virginia and Missouri, Seattle and Portland | R-60 |
| 5 | Southern Great Lakes and lower New England - Chicago, Boston, Denver | R-60 |
| 6 | Upper Midwest and northern New England - Minneapolis, Burlington | R-60 |
| 7 | Coldest lower-48 pockets - Duluth, northern North Dakota, high mountains | R-60 |
Not sure which zone you’re in? The IECC assigns zones by county - look yours up on the DOE/IECC climate zone map. And note these are the current model-code levels: many states still enforce an older IECC edition with lower floors (R-38 and R-49 were common), so the table above may be more than your local minimum. Exceeding a code minimum is never a problem; falling short of it is.
What those R-values look like in inches
R-value is just the material’s R per inch times the depth, so the same target means very different depths depending on what’s up there:
| Material | R per inch | R-30 | R-49 | R-60 |
|---|---|---|---|---|
| Blown fiberglass | R-2.5 | 12.0 in | 19.6 in | 24.0 in |
| Fiberglass batt | R-3.2 | 9.4 in | 15.3 in | 18.8 in |
| Blown cellulose | R-3.5 | 8.6 in | 14.0 in | 17.1 in |
| Mineral wool | R-4.0 | 7.5 in | 12.3 in | 15.0 in |
| Open-cell spray foam | R-3.6 | 8.3 in | 13.6 in | 16.7 in |
| Closed-cell spray foam | R-6.5 | 4.6 in | 7.5 in | 9.2 in |
So a zone 5 attic at R-60 wants roughly 17 in (43 cm) of cellulose or 24 in (61 cm) of blown fiberglass - much deeper than most people expect, which is why so many attics measure in at half the recommended level.
Worked example: topping up a zone 5 attic
Say you’re in Chicago (zone 5) and the attic has 6 inches of old blown fiberglass:
- Current R: 6 in × R-2.5/in = R-15.
- Target for zones 4-7: R-60. Gap: 60 - 15 = R-45 to add.
- Top up with blown cellulose (R-3.5/in): 45 ÷ 3.5 = 12.9 in of new cellulose blown right over the old fill.
- Final depth: about 6 + 13 = 19 in (48 cm) total.
Layers add - you don’t remove the old insulation, you bury it (as long as it’s dry and mold-free).
👉 The R-Value Calculator does this instantly - pick a material and thickness, choose Attic and your climate zone, and it shows the assembly R-value and whether it meets, nearly meets, or misses the recommended level. It covers walls (R-13 to R-21) and floors (R-13 to R-38) by zone too.
Watch-outs
- Air-seal before you insulate. Insulation slows heat flow but doesn’t stop air leaks; seal top plates, wire and pipe penetrations, and the attic hatch first, or warm moist air will bypass all that new R-value.
- Don’t compress batts. A batt squashed below its rated thickness loses the trapped air that does the insulating - an R-19 batt forced into a 2x4 wall delivers only about R-14.
- Keep soffit vents open. Install baffles at the eaves so deep loose fill doesn’t choke the ventilation path.
- Check recessed lights. Only IC-rated (insulation-contact) fixtures may be buried; older cans need clearance or replacement.
- Measure settled depth, not poured depth. Loose fill settles over time - the R that counts is today’s actual depth times R per inch.
Related
- R-Value Calculator - assembly R-value from material and thickness, checked against your zone’s recommended level.
- Walls & Ceilings tools - the full walls and insulation toolset.
- How thick is R-30 insulation - the depth-per-material math in detail.
- Minimum roof pitch for shingles - the roof above that attic, worked out the same way.