ASHRAE / IECC Climate Zones Explained (Zones 1–8, A/B/C)
How the ASHRAE 169 / IECC climate zone map works — thermal zones 1 (hottest) to 8 (coldest), moist/dry/marine (A/B/C) designations, and what the zone drives in building design.
ASHRAE / IECC Climate Zones Explained (Zones 1–8, A/B/C)
Direct answer
The IECC / ASHRAE 169 climate zone system — the eight thermal zones, the A/B/C moisture designations, and what each drives in envelope and HVAC design.
Key facts
| Metric | Value | Notes |
|---|---|---|
| Thermal zones | 1–8 | 1 hottest, 8 coldest |
| Moisture regimes | A / B / C | moist / dry / marine |
| Zone basis | heating & cooling degree-days | plus precipitation |
| Assigned by | county | per code climate zone map |
What the climate zone system is
The climate zone map used by the U.S. energy codes comes from ASHRAE Standard 169, and the same map underlies both the IECC (used for most residential and much commercial work) and ASHRAE 90.1 (the commercial alternative). It divides the country into eight thermal zones — from Zone 1 (hottest) to Zone 8 (coldest, arctic) — based primarily on heating and cooling degree-days. A moisture designation is layered on top.
The eight thermal zones
| Zone | Thermal character | Typical U.S. examples | Envelope emphasis |
|---|---|---|---|
| 1 | Very hot | South Florida, Hawaii, Puerto Rico | Cooling + solar control; low SHGC |
| 2 | Hot | Houston, Phoenix, Tampa | Cooling-dominated; shading, low SHGC |
| 3 | Warm | Atlanta, Los Angeles, Dallas | Balanced, cooling-leaning |
| 4 | Mixed | Baltimore, St. Louis, Seattle (4C) | Balanced heating/cooling |
| 5 | Cool | Chicago, Denver, Boston | Heating-leaning; more wall/roof R |
| 6 | Cold | Minneapolis, Burlington, Helena | Heating-dominated; high R, low U |
| 7 | Very cold | Duluth, northern Maine, interior Alaska | High R, thermal-bridge control |
| 8 | Subarctic / arctic | Fairbanks, northern Alaska | Maximum R; continuous insulation |
The A / B / C moisture designations
Each thermal zone (through Zone 6) also carries a moisture regime:
- A — Moist (e.g. most of the eastern U.S.)
- B — Dry (e.g. the interior Southwest and Mountain West)
- C — Marine (e.g. the Pacific coast, defined by mild, wet conditions)
So a full designation looks like 4A (mixed-humid), 3B (hot-dry), or 4C (marine). The suffix drives vapor-control strategy — where and whether to place a vapor retarder — and occasionally which prescriptive path applies. Zones 7 and 8 have no suffix.
Why it matters at schematic design
The zone is the single input that unlocks the rest of the prescriptive energy code: insulation minimums, fenestration limits, and air-sealing expectations all key off it. Establishing the correct zone — and confirming the exact code edition your AHJ has adopted — should happen before any schematic envelope decision, because it sets the targets everything else is measured against.
Set the zone before you design the envelope
In Harth, fix the project's climate zone at the start of a schematic study so envelope R-values, glazing U-factors, and SHGC targets are anchored to the right code baseline from the first massing move.
Frequently asked questions
What do the A, B, and C suffixes mean?
They describe the moisture regime, not temperature. A is moist, B is dry, and C is marine. So Zone 4A (mixed-humid, e.g. much of the mid-Atlantic) differs from 4B (dry) and 4C (marine, e.g. the Pacific Northwest coast) in how you handle vapor control, and in some cases which prescriptive envelope path applies. Zones 7 and 8 are cold enough that they carry no moisture suffix.
How is a building's climate zone determined?
It is assigned by county in the climate zone map of the adopted energy code (IECC or ASHRAE 90.1, both derived from ASHRAE Standard 169). You do not calculate it from your own weather data — you look up the county. Some states amend the map or reassign specific counties, so confirm against the edition your AHJ enforces.
What does the climate zone actually control?
It is the master key to the prescriptive energy code: it sets minimum insulation R-values, maximum fenestration U-factors and solar heat gain coefficients (SHGC), air-sealing expectations, and which HVAC and water-heating efficiency provisions apply. Get the zone wrong and every downstream envelope target is wrong.
Source & method: Structure of the ASHRAE Standard 169 / IECC climate zone map. Zone descriptors are summarized for orientation; the county-level zone assignment and any local amendments in the AHJ-adopted energy code edition are authoritative. Values shown are typical/illustrative, not reproduced verbatim from code tables.