Quick answer
Your climate zone comes from your county: look it up by ZIP code, city or county in the climate zone lookup. Under the 2021 International Energy Conservation Code, attics need R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8. R-60 is about 24 inches of blown fiberglass or 19 inches of blown cellulose.
How climate zones work
The International Energy Conservation Code (IECC) assigns every US county a zone from 1, the hottest, to 8, the coldest. Most zones also carry a letter for moisture: A for moist, B for dry and C for marine. Zone 4C covers the mild, marine counties west of the Cascades in Washington and Oregon, plus Del Norte and Humboldt counties in California. The zone sets the minimum insulation, window ratings and other energy rules for new homes and additions, and it is a good guide for upgrades too.
To find yours, enter a ZIP code, city or county in the climate zone lookup, or open your state from the climate zones by state page, which lists every county.
Attic insulation by climate zone
Minimum ceiling (attic) R-values in the last two code editions, and ENERGY STAR's recommendation for an existing home:
| Zone | Climate | 2021 IECC | 2018 IECC | ENERGY STAR, existing home |
|---|---|---|---|---|
| Zone 1 | Very hot, humid | R-30 | R-30 | R-30 for an uninsulated attic, or add R-25 if you already have 3–4 inches |
| Zone 2 | Hot | R-49 | R-38 | R-49 for an uninsulated attic, or add R-38 if you already have 3–4 inches |
| Zone 3 | Warm | R-49 | R-38 | R-49 for an uninsulated attic, or add R-38 if you already have 3–4 inches |
| Zone 4 | Mixed | R-60 | R-49 | R-60 for an uninsulated attic, or add R-49 if you already have 3–4 inches |
| Zone 5 | Cool | R-60 | R-49 | R-60 for an uninsulated attic, or add R-49 if you already have 3–4 inches |
| Zone 6 | Cold | R-60 | R-49 | R-60 for an uninsulated attic, or add R-49 if you already have 3–4 inches |
| Zone 7 | Very cold | R-60 | R-49 | R-60 for an uninsulated attic, or add R-49 if you already have 3–4 inches |
| Zone 8 | Subarctic | R-60 | R-49 | R-60 for an uninsulated attic, or add R-49 if you already have 3–4 inches |
Marine zone 4C uses the zone 5 row of the code table; for attics that changes nothing, since zones 4 and 5 both need R-60. Your town may still enforce the 2018 or an earlier edition for permits; the energy code by state page shows what each state has adopted.
How many inches that takes
Blown insulation is rated per inch. We use R-2.5 per inch for blown fiberglass and R-3.2 for blown cellulose, inside the US Department of Energy's ranges (R-2.2 to 2.7 and R-2.9 to 3.8). A lighter product needs more depth, so check the bag:
| Target | Blown fiberglass | Blown cellulose |
|---|---|---|
| R-30 | about 12 in | about 9 in |
| R-38 | about 15 in | about 12 in |
| R-49 | about 20 in | about 15 in |
| R-60 | about 24 in | about 19 in |
The bag label gives the exact depth and coverage for each R-value, and it is the number to follow. If the attic already has insulation, measure its depth and count only what you need to add: the insulation calculator works out the top-up and the bags.
For example, say you're in zone 5 with 6 inches of old loose fiberglass, about R-15. Reaching R-60 means adding R-45: about 18 inches of blown fiberglass or 14 inches of cellulose.
Before you add insulation
- Seal air leaks into the attic first, around light fixtures, plumbing and wiring holes, and the attic hatch. Insulation does little against air that flows through it.
- Keep the soffit vents open with baffles so air can still move under the roof deck.
- If the attic has old vermiculite (gray-brown, pebbly loose fill), don't disturb it: it may contain asbestos, so have it tested first. Have an electrician check any live knob-and-tube wiring before you cover it.
- Keep insulation away from recessed lights that are not rated for contact with it, and from chimneys and flues, as the manufacturer and your local code require.
Frequently asked questions
How do I find my climate zone?
Climate zones are set by county. Enter your ZIP code, city or county in the climate zone lookup, or find your county on your state's climate zone page.
How much attic insulation do I need in zone 4 or 5?
R-60 under the 2021 IECC, about 24 inches of blown fiberglass or 19 inches of blown cellulose. The 2018 edition asked for R-49.
How much attic insulation do I need in zone 2 or 3?
R-49 under the 2021 IECC, about 20 inches of blown fiberglass or 15 inches of blown cellulose.
What do the letters A, B and C mean?
Moisture: A is moist, B is dry and C is marine. For attic insulation the letter makes no difference. Marine zone 4C follows the zone 5 row of the code table, which changes its wall, floor and basement figures but not the attic.
Calculators for this
What Climate Zone Am I In?
Find your IECC climate zone by ZIP code, city or county, and the insulation R-values the energy code requires for your attic, walls, floors and basement.
Insulation
Get batt bags for walls, floors and attics, or bags of blown-in fiberglass or cellulose to reach your target R-value, by attic size.
Heating Cost Comparison
Compare the cost of heat from natural gas, propane, heating oil, electric heat and a heat pump, per million Btu and for a whole winter, with your own prices.
Published 2026-11-04. Sources: 2021 International Energy Conservation Code, Table R402.1.3; ENERGY STAR, Recommended home insulation R-values; US Department of Energy, Insulation materials (archived copy; DOE removed the page in 2026). See how we calculate.