ASHRAE 169 climate zones explained: zones 0 to 8 and moisture regimes A, B and C

What is ASHRAE Standard 169?

ANSI/ASHRAE Standard 169, Climatic Data for Building Design Standards, is the source of climate data behind ASHRAE's building standards. It is referenced by Standards 90.1, 90.2, 90.4, 100, 127 and 189.1, and it contains design conditions and climate zones for weather stations worldwide. Its 2021 update (Addendum a to 169-2020) covers 9,237 locations and is compiled from the 2021 ASHRAE Handbook of Fundamentals. The 2025 edition, updated from the 2025 ASHRAE Handbook—Fundamentals, lists 12,424 stations.

The 2013 edition redefined the zones on more recent weather data and added zone 0 for extremely hot climates. In the United States, about 400 of more than 3,000 counties were reassigned (according to PNNL's analysis), most of them to warmer zones.

What do the zone numbers 0 to 8 mean?

The thermal zone depends only on degree days: heating degree days with an 18 °C base (HDD18, or HDD65 in °F) and cooling degree days with a 10 °C base (CDD10, or CDD50 in °F). In SI units, as set out in Table C301.3 of the 2021 IECC:

• 0, extremely hot: CDD10 above 6000.
• 1, very hot: CDD10 between 5000 and 6000.
• 2, hot: CDD10 above 3500, up to 5000.
• 3, warm: CDD10 below 3500 and HDD18 up to 2000.
• 4, mixed: CDD10 below 3500 and HDD18 above 2000, up to 3000.
• 5, cool: CDD10 below 3500 and HDD18 above 3000, up to 4000.
• 6, cold: HDD18 above 4000, up to 5000.
• 7, very cold: HDD18 above 5000, up to 7000.
• 8, subarctic and arctic: HDD18 above 7000.

What do the letters A, B and C mean?

There are 19 combinations in total. A and B apply to zones 0 to 6, C (marine) only to zones 3, 4 and 5, and zones 7 and 8 have no letter; the IECC notes that a missing letter means the moisture regime is irrelevant.

• C, marine: coldest-month mean between −3 °C and 18 °C, warmest-month mean below 22 °C, at least four months above 10 °C, and a dry summer, meaning the wettest month of the cold season gets at least three times the rain of the driest month of the rest of the year.
• B, dry: not marine, and annual precipitation below a threshold that rises with annual mean temperature and depends on how much of the rain falls in the high-sun half of the year (April to September in the Northern Hemisphere).
• A, humid (also called moist): everything that is neither marine nor dry.

How do you find a location's ASHRAE climate zone?

For listed locations, use the tables: Standard 169 assigns zones to weather stations worldwide and to every US county, and the IECC reproduces the county list. For other places, calculate HDD18 and CDD10 from long-term data and apply the thresholds above, then set the moisture regime from monthly temperature and precipitation. When only annual data is available, the IECC gives shortcuts; for example, a zone 3 location with CDD10 of 2500 or less is 3C.

ASHRAE's climate zone maps are illustrative. In the 2021 update they were computed from ERA5 reanalysis at 0.25° for 1994–2019, and the standard states that the station data in its normative appendix takes precedence.

Station choice matters. In the 2021 tables, London Heathrow is 4A while London Luton and Stansted are 5A, and Stockholm Arlanda is 6A while Stockholm Observatoriet is 5A. Barcelona airport is 3A and Madrid-Barajas is 3B.

Where are ASHRAE 169 zones used in design?

In the United States, the zone decides which prescriptive requirements apply under ASHRAE 90.1 and the IECC; insulation requirements, for example, vary by zone. The US Department of Energy's analysis of ASHRAE 90.1-2019 simulates one representative city for each of the 16 zones found in the country, from Honolulu (1A, very hot and humid) and Tucson (2B) to Seattle (4C), Denver (5B) and Fairbanks (8).

Because Standard 169 lists stations worldwide, the zones also give international teams a common reference for comparing sites.

For design, the zone is a starting point rather than a climate analysis. It reduces a year to two degree-day totals and a moisture label, so it cannot show when heat and humidity coincide, how much sun a façade receives or how windy the site is.

How C4B shows ASHRAE 169 zones

C4B shows each location's ANSI/ASHRAE 169 climate zone on the Climate Data tab, next to its Köppen-Geiger class and its heating and cooling degree days at an 18.3 °C base. It is available on every plan, including Free.