Where these numbers come from
The table above shows the levels ENERGY STAR recommends when you add insulation to an existing home. ENERGY STAR's table is based on Table R402.1.3 of the 2021 International Energy Conservation Code (IECC), the same minimums DOE lists for new homes.
A few things to know when you read it:
- These are insulation R-values. They're the nominal R printed on the batt or bag, not the R of the whole wall or ceiling with framing included.
- Walls and basement walls show the cavity-only option. The code also accepts combinations of cavity insulation plus continuous foam board. See the PNNL comparison of IECC requirements for those.
- Zone 0 shares zone 1's levels.
- No basement wall level is published for zones 1 and 2. The calculator shows "no target" if you pick one of those zones for basement walls.
How to find your climate zone
The IECC splits the country into numbered zones, from the hottest (0 and 1) to the coldest (8), and adds a letter for moisture: A (moist), B (dry) or C (marine). Zones are assigned by county.
When you enter a ZIP code, the calculator matches it to its county using the Census Bureau's ZIP Code Tabulation Areas, then looks up that county's zone in PNNL-33270, PNNL's county-by-county guide to the 2021 IECC zones. If your ZIP isn't found (some PO-box-only ZIPs aren't in the Census data), pick your zone from the list.
Use your ZIP rather than your state. In our data, Texas spans six zones and California seven, from hot-humid coast to cold mountains.
Worked example: the same attic in different zones
To see what the zone changes, take a 1,000 sq ft attic with 3.5 in of old fiberglass batts (R-11) and top it up with blown cellulose. With 10% waste:
| Zone | Attic target | Blow in | Settles to | Bags | Material cost |
|---|---|---|---|---|---|
| 1 | R-30 | 5.64 in | 5.24 in | 25 | $400–$449 |
| 2–3 | R-49 | 11.1 in | 10.3 in | 58 | $927–$1,042 |
| 4–8 | R-60 | 14.2 in | 13.2 in | 78 | $1,247–$1,402 |
The same attic needs three times as many bags in a cold zone as in zone 1, because the gap between what's there and the target is so much bigger. From zone 4 up, the attic target stops rising: zones 4 through 8 all recommend R-60.
Reading the table by area
Attic
R-30, R-49 or R-60 on the attic floor. This is usually the easiest place to reach the target, because there's room to pile insulation as deep as you need. Use the attic insulation calculator.
Walls
R-13 in zones 1–2, R-20 in zone 3 and R-30 in zones 4–8. A 2×4 wall holds about R-13 to R-15 of batts and a 2×6 holds about R-21, so R-30 isn't reachable with batts alone. The wall insulation calculator shows how much fits and how much continuous board would close the gap.
Floors
Floors over a crawl space, garage or unheated basement: R-13 in zones 1–2, R-19 in zones 3–4, R-30 in zones 4C, 5 and 6, and R-38 in zones 7–8. See the floor insulation calculator.
Basement walls
R-13 in zones 3–4 (except 4C) and R-19 in zone 4C and zones 5–8. The basement insulation calculator covers foam board and framed walls.
Code minimums and local rules
The IECC is a model code. States and cities adopt it on their own schedule, sometimes an older edition and sometimes with amendments. If your project needs a permit, your building department's rules apply, and they may differ from this table. For a project without a permit, these levels are a sound target. To see how thick each level is in your chosen material, use the R-value chart.
Sources
- ENERGY STAR: Recommended home insulation R-values
- U.S. DOE: Insulation and air-sealing essentials (2021 IECC minimum R-values)
- PNNL Building America Solution Center: 2009–2021 IECC minimum insulation requirements
- PNNL-33270: Guide to Determining Climate Zone by County (IECC 2021)
- US Census Bureau: 2020 ZCTA to County Relationship File