Types

IECC and ASHRAE thermal zones

• Zones 0 to 8 by heating and cooling degree days, plus a moisture letter: A (moist), B (dry), C (marine)
• Used for envelope insulation, fenestration, and energy modeling inputs

Building America regions

• Eight regions: hot-humid, hot-dry, mixed-humid, mixed-dry, marine, cold, very cold, subarctic
• Defined by precipitation, temperature, and heating degree days; boundaries follow county lines
• Used for moisture and construction practice guidance

Koppen-Geiger

• Wladimir Koppen published it in 1884, revised in 1918 and 1936; Rudolf Geiger revised it in 1954 and 1961
• Five groups: A tropical (every month 64 F or higher), B arid, C temperate, D continental, E polar
• Global; a 2018 update maps present and future climate at 1 km resolution
• Used for regional and international comparison

USDA Plant Hardiness Zones

• 13 zones, each 10 F wide, split into A and B half-zones of 5 F, based on average annual extreme minimum winter temperature
• 2023 map released November 15, 2023, from 1991 to 2020 data at 13,412 stations (7,983 for the 2012 map)
• About half of the country moved to the next warmer half-zone, generally 0 to 5 F
• ZIP code lookup at the USDA site; used for plant selection, not building envelope

EPA ecoregions

• Level I: 15 North American regions; Level II: 50; Level III: 182
• Based on ecosystem and vegetation patterns
• Used for native planting and habitat context

Standards

Thermal zone criteria

Degree days: CDD50 and HDD65 in F-days (SI uses CDD10 and HDD18 in C-days).

Moisture regimes

• Marine (C): all of: coldest month mean 27 F to 65 F; warmest month mean below 72 F; at least 4 months above 50 F; wettest cold-season month at least 3 times the driest warm-season month (cold season is October to March in the Northern Hemisphere)
• Marine shortcut from degree-day data: zone 3 with CDD50 4,500 or less, zone 4 with 2,700 or less, zone 5 with 1,800 or less
• Dry (B): not marine, and annual precipitation below a threshold that depends on mean annual temperature and the share of precipitation falling April to September (70 percent or more, 30 to 70 percent, 30 percent or less each use a different equation)
• Humid (A): neither marine nor dry

Representative cities

Zones are from ANSI/ASHRAE Standard 169-2020, Addendum a (October 29, 2021). Zone assignments change between editions, so cite the edition with the zone.

Selection

• Envelope and energy: IECC and ASHRAE zone for the project county
• Moisture and assembly practice: Building America region, then the A, B, C letter
• Planting: hardiness zone for cold tolerance; ecoregion for native plant communities
• Cross-border or international work: Koppen group
• Confirm the zone for the project location against the adopted code or standard edition

Sources

• ASHRAE Meteo: climate zone definitions for IECC and ASHRAE, https://ashrae-meteo.info/v2.0/help_IECC.php
• U.S. Department of Energy: Climate zones, https://www.energy.gov/eere/buildings/climate-zones
• ASHRAE: ANSI/ASHRAE Addendum a to Standard 169-2020, Climatic Data for Building Design Standards (October 29, 2021), Tables A-3 to A-6, Infrastructure/PDF resources/169_2020_a_20211029.pdf
• USDA Agricultural Research Service: USDA unveils updated Plant Hardiness Zone Map, https://www.ars.usda.gov/news-events/news/research-news/2023/usda-unveils-updated-plant-hardiness-zone-map/
• USDA: Plant Hardiness Zone Map, https://planthardiness.ars.usda.gov/
• U.S. EPA: Ecoregions of North America, https://www.epa.gov/eco-research/ecoregions-north-america
• Wikipedia: Koppen climate classification, https://en.wikipedia.org/wiki/Köppen_climate_classification
• Wikimedia Commons: United States Köppen.png, by Adam Peterson (CC BY-SA 4.0), https://commons.wikimedia.org/wiki/File:United_States_Köppen.png