What R-value for attic insulation?
Reviewed 2026-07-04 · IRC 2021 minimums and Energy Star targets
Attic targets by climate zone: R30–R38 in the hot South, R49 across the middle of the US, R60 in the far North (IRC 2021 minimums). Energy Star recommends R60 for most of zones 4–8. In inches: R49 is about 14″ of blown cellulose or 16–18″ of blown fiberglass. If you can see your joists, you're under target.
R-value by climate zone
| Where you live | IRC 2021 minimum | Energy Star target |
|---|---|---|
| Zone 1–2 (FL, south TX, AZ low desert) | R30–R38 | R38–R49 |
| Zone 3 (Southeast, coastal CA) | R38 | R49 |
| Zone 4 (Mid-Atlantic, lower Midwest) | R49 | R60 |
| Zone 5 (Chicago, Denver, Boston) | R49 | R60 |
| Zone 6 (MN, WI, northern New England) | R49 | R60 |
| Zone 7–8 (northern MN, mountains, AK) | R60 | R60 |
What do I have, and what do I add?
Measure with a ruler at 3 or 4 spots; use the average.
Top-up assumes blown cellulose at R3.5 per inch over the existing layer. Old material stays unless wet, moldy, or covering knob and tube wiring.
Frequently asked
What R-value do I need for my attic?
By IRC 2021 code minimums: R30 to R38 in the hot South (zones 1 to 3), R49 across the middle of the country (zones 4 to 6), R60 in the far North. Energy Star recommends going one step higher, R60 for most of zones 4 through 8, because attic insulation is cheap relative to what it saves.
How many inches of blown insulation is R49?
Roughly 14 inches of blown cellulose (R3.5 per inch) or 16 to 18 inches of blown fiberglass (R2.5 to 3.0 per inch settled). If you can see your ceiling joists, you are usually somewhere under R19 and well below any current target.
Can I add new insulation on top of old?
Yes, and it is the normal retrofit. Blow loose fill directly over existing material (unfaced only on top; never put a vapor barrier between layers). Two exceptions: wet or moldy insulation comes out first, and pre-1950s knob and tube wiring must never be buried, that is a documented fire risk.
Is more attic insulation always better?
Returns diminish. Going from R19 to R49 is a big, fast payback. Going from R49 to R60 is a small improvement that mainly makes sense in cold zones or while the blower is already rented. Past R60, put the money into air sealing instead, leaks undo insulation faster than thickness fixes it.
Related tools
Sources
- IRC 2021 Table N1102.1.3 ceiling R-value minimums by climate zone.
- ENERGY STAR recommended home insulation R-values (attic: R60 guidance for zones 4–8 new construction).
- R-per-inch by material: manufacturer technical data (blown fiberglass ~R2.5–3.0 settled, cellulose ~R3.2–3.8, batts ~R3.1–3.4).
- Knob and tube prohibition: NEC Article 394.