One house, two climates
Picture the same small cabin built twice: once near Miami, once near Duluth.
- In Miami, the main problem is heat and humidity. Shade and dry air matter most.
- In Duluth, the main problem is long, cold winters. Insulation and air sealing matter most.
The walls, windows, water tank, solar array and garden that work in one place can fail in the other. That is why this course never gives one number for everyone. Climate decides.
What a climate zone is
A climate zone is a region with similar weather over a typical year. In the US, building codes use the IECC climate zones. IECC stands for the International Energy Conservation Code, the model energy code many states adopt.
Each zone has two parts:
- A number from 1 to 8, from hottest to coldest. Zone 1 covers places like south Florida and Hawaii. Zone 8 covers the coldest parts of Alaska.
- A moisture letter: A for moist, B for dry, C for marine.
So 5A means cold and moist, like much of the upper Midwest. 2B means hot and dry, like low desert areas of Arizona.
How the number is set
The zone number comes mostly from degree days.
- Heating degree days (HDD) add up how far, and for how long, the daily average temperature falls below 65 °F (18 °C). More HDD means more heating.
- Cooling degree days (CDD) add up how far it rises above 65 °F. More CDD means more cooling.
A day that averages 45 °F adds 20 heating degree days. A cold place piles up thousands of HDD a year. A hot place piles up thousands of CDD instead.
Text description
A chart of daily average temperature through an example year in a cold US place. The temperature axis is marked at 25, 45, 65 and 85 °F. A horizontal line marks the 65 °F (18 °C) base. In winter the temperature curve falls well below the line; the area between them is shaded and stands for heating degree days. In summer the curve rises a little above the line; that smaller shaded area stands for cooling degree days. A marker on a fall day shows that a day averaging 45 °F adds 20 heating degree days, because 65 minus 45 is 20.
Degree days come from long-term weather records, such as NOAA climate normals, so they describe a typical year, not a single one.
What the letters mean
The moisture letter tells you about rain and humidity.
- A, moist: most of the eastern US. Humid summers. Mold and moisture in walls are common problems.
- B, dry: much of the interior West. Low humidity, big swings between day and night, and often little rain for catchment.
- C, marine: a narrow band along the Pacific coast. Mild, damp winters and dry summers.
Zones 7 and 8 have no letter. They are cold enough that cold sets the rules.
Text description
A schematic grid, not a map. Rows are zone numbers from 8 (coldest) at the top to 1 (hottest) at the bottom. Columns are moisture regions: C, marine, along the Pacific coast; B, dry, in the interior West; and A, moist, in most of the East. Cells: zone 8, the coldest parts of Alaska, and zone 7 have no letter, because cold sets the rules. Zone 6 has 6B and 6A. Zone 5 has 5B, such as Denver, and 5A, such as Boston and the upper Midwest. Zone 4 has 4C, such as Seattle, 4B and 4A. Zone 3 has 3C, such as San Francisco, 3B, such as Las Vegas, and 3A, such as Atlanta. Zone 2 has 2B, such as Phoenix and the low desert, and 2A. Zone 1 has 1A, such as Miami and Hawaii. A note says real zones are set by county and to check the IECC climate zone map.
Moist, dry or marine?
Sort each place by its IECC moisture letter.
- Las Vegas, Nevada
- Miami, Florida
- Denver, Colorado
- Seattle, Washington
- Atlanta, Georgia
- Boston, Massachusetts
- San Francisco, California
- Phoenix, Arizona
How zone changes each budget
- Heat: colder zones need much more insulation and tighter air sealing. Hot zones need shade and good control of summer sun.
- Energy: colder, cloudier zones have shorter winter days, so solar makes less when you need it most.
- Water: dry zones (B) collect less rain and lose more to evaporation. Cold zones must protect pipes and tanks from freezing.
- Food: the growing season is longer in warm zones. For plants, gardeners also use the USDA plant hardiness zone, which is based on the coldest winter temperatures. It is a different map from the IECC one.
Finding your zone
Zones are assigned by county. To find yours:
- Look up the IECC climate zone map through the US Department of Energy’s Building America Solution Center or Building Energy Codes Program.
- Or ask your local building department. They will know which zone and which code edition they use.
Check
A place is listed as zone 6A. What does that tell you?
Summary
- IECC climate zones run from 1 (hottest) to 8 (coldest).
- A moisture letter follows: A moist, B dry, C marine. Zones 7 and 8 have none.
- The number comes mostly from heating and cooling degree days.
- Zones are set by county. Your building department can confirm yours.
- Climate zone is an input in every project and tool in this course.
Next, you will zoom in from the region to your own piece of land.
Go deeper: microclimates and changing climates
A zone describes a region, but your site has its own microclimate. A south-facing slope can be warmer than the valley below it. A hollow can collect cold air and frost. Cities run warmer than the countryside. Zone maps are also updated as editions of the code change and as long-term weather records are revised. Since 2021, the IECC also defines a zone 0 for extremely hot climates. No part of the US falls in it. Treat your zone as a starting point, then watch your own site.