IECC Climate Zones
Eight zones, three moisture regimes, one set of degree-day thresholds that every U.S. energy code, Manual J load calculation, and prescriptive envelope R-value table leans on.
Zones 1–8 — degree-day ranges
| Zone | Name | HDD65°F | CDD50°F | Typical climate |
|---|---|---|---|---|
| 1 | Very hot | — | > 9,000 | Tropical / subtropical |
| 2 | Hot | ≤ 3,000 | 6,300–9,000 | Hot humid or hot dry |
| 3 | Warm | ≤ 3,600 | 4,500–6,300 | Warm humid / warm dry / marine |
| 4 | Mixed | 3,600–5,400 | — | Mixed humid / mixed dry / marine |
| 5 | Cool | 5,400–7,200 | — | Cool humid / cool dry / marine |
| 6 | Cold | 7,200–9,000 | — | Cold humid / cold dry |
| 7 | Very cold | 9,000–12,600 | — | Subarctic |
| 8 | Subarctic / arctic | > 12,600 | — | Arctic |
Degree-day values in °F·days. Zones 1–3 are bounded by cooling degree days; zones 4–8 by heating degree days. Paraphrased from ASHRAE 169 and IECC Table R301.1 — use the code itself for compliance.
Moisture regimes — A, B, C
| Regime | Climate | Defining condition |
|---|---|---|
| A | Moist | Applied to any zone not classified B or C. Typical east-of-Mississippi humid climates. |
| B | Dry | Low annual precipitation threshold tied to mean annual temperature (Köppen BWk/BSk). Great Plains, Mountain West, desert Southwest. |
| C | Marine | All months above 27 °F mean, warmest month below 72 °F mean, ≥4 months above 50 °F, and winter-biased precipitation. Applies in zones 3 & 4 on the U.S. West Coast. |
U.S. city examples
| Zone | Example cities |
|---|---|
| 1A | Miami FL · Honolulu HI · Key West FL · San Juan PR |
| 2A | Houston TX · New Orleans LA · Orlando FL · Jacksonville FL · Charleston SC |
| 2B | Phoenix AZ · Tucson AZ · Yuma AZ |
| 3A | Atlanta GA · Memphis TN · Dallas TX · Oklahoma City OK · Little Rock AR |
| 3B | Los Angeles CA · Las Vegas NV · Albuquerque NM (lower elev.) · El Paso TX |
| 3C | San Francisco CA · San Jose CA · Monterey CA |
| 4A | Baltimore MD · Philadelphia PA · Washington DC · Nashville TN · St. Louis MO · Kansas City MO |
| 4B | Albuquerque NM (upper elev.) · Roswell NM |
| 4C | Seattle WA · Portland OR · Salem OR · Tacoma WA |
| 5A | Chicago IL · New York NY · Boston MA · Denver CO · Cleveland OH · Detroit MI |
| 5B | Boise ID · Reno NV · Salt Lake City UT · Colorado Springs CO |
| 6A | Minneapolis MN · Burlington VT · Portland ME · Buffalo NY · Milwaukee WI |
| 6B | Helena MT · Rapid City SD · Cheyenne WY · Great Falls MT |
| 7 | Duluth MN · International Falls MN · Aspen CO · Anchorage AK |
| 8 | Fairbanks AK · Barrow AK · Nome AK |
Examples selected from IECC Figure C301.1 and ASHRAE 169 informative annex. County-level zone assignments vary — always verify using the IECC county map for your jurisdiction before applying to code compliance.
Using the zone in practice
- Prescriptive path. IECC R402.1 / C402.1 lists minimum insulation and fenestration values by zone — often the fastest path to code compliance for residential and small commercial.
- Performance path. Energy-model the building (REM/Rate, EnergyPlus, IES-VE) and show it beats the prescriptive zone baseline on energy cost or source energy.
- Manual J. ACCA Manual J uses ASHRAE Chapter 14 outdoor design temperatures (99% heating, 1% cooling) keyed by location — the climate zone is a coarse proxy but load calculations use the specific station design temps.
- Moisture control. Zones 4–8 Class-I/II vapor retarder requirements and cladding rain-screen detailing change at zone boundaries. Marine C zones have distinct wall assembly rules.
Sources
FAQ
What HDD65 range defines IECC climate zone 5?
Per the CalcSpec IECC/ASHRAE 169 reference, climate zone 5 (Cool) spans 5,400 to 7,200 HDD65 in °F·days. Zones 4 through 8 are bounded by heating degree days, while zones 1 through 3 are bounded by cooling degree days (CDD50).
Which IECC climate zone is Chicago in?
Chicago IL is in IECC climate zone 5A, per the CalcSpec city-example table. Zone 5A (Cool, Moist) also includes New York NY, Boston MA, Denver CO, Cleveland OH, and Detroit MI. City examples are drawn from IECC Figure C301.1 and the ASHRAE 169 informative annex.
How is the marine (C) moisture regime defined?
Per ASHRAE 169 as summarized by CalcSpec, the marine (C) regime requires all months above 27°F mean, the warmest month below 72°F mean, at least 4 months above 50°F, and winter-biased precipitation. It applies in zones 3 and 4 on the U.S. West Coast.
What are the degree-day thresholds for the coldest IECC zones, 7 and 8?
Per the CalcSpec IECC/ASHRAE 169 reference, zone 7 (Very cold, Subarctic) spans 9,000 to 12,600 HDD65, and zone 8 (Subarctic/arctic, Arctic) is above 12,600 HDD65, in °F·days. Values paraphrase ASHRAE 169 and IECC Table R301.1.
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Related
Degree-day boundaries, moisture regimes, and city examples paraphrased from ASHRAE 169 and 2021 IECC for reference. Copyrighted standard text not reproduced. Reference only; not code-compliance guidance.