Every three years, the International Code Council (ICC) updates the building codes in the International Energy Conservation Code (IECC). Changes to the IECC come from ICC staff, industry groups, government, and the general public. The IECC is the model energy code in the U.S., and updates to the 2021 edition were finalized by ICC in December 2020. One of the fundamental changes to the 2021 IECC was the designation of Climate Zones (CZ).

Climate zones are central to the IECC. Climate zones dictate many of the energy efficiency measures that a building must include, and they are especially relevant to the building envelope. Climate zones are defined at the county level and are based on weather factors like winter and summer temperatures along with humidity and rainfall (to define the "Dry" and "Marine" sub-climates). The climate zone map had not changed since the 2003 IECC.

However, with new research based on measured temperature data from over 4000 weather stations throughout North America over the last 25 years, the IECC designated changes to the CZ map for the first time in nearly 20 years. The outcome was that about 10% of counties in the U.S. were placed in a new CZ. In nearly all cases, the shift was to a warmer (lower) CZ, reflecting a general warming of the climate in those areas.

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