Iowa climate zones and HVAC design temperatures
Iowa spans 2 IECC climate zones. 89% of people live in zone 5A (cool, humid). In Des Moines, heating and cooling equipment is sized for 90°F in summer and 0°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Des Moines | 5A | 90°F | 0°F | 53°F |
| Cedar Rapids | 5A | 88°F | -4°F | 52°F |
| Davenport | 5A | 88°F | 0°F | 53°F |
| Sioux City | 5A | 90°F | -1°F | 52°F |
| Iowa City | 5A | 90°F | 0°F | 54°F |
Summer: exceeded in 1% of hours. Winter: colder in only 1% of hours. Water in: estimated groundwater temperature for tankless water heater sizing. Computed for each county from NOAA hourly weather data, 2015-2024; see where our numbers come from. Hottest summer design in the state: 92°F (Harrison County); coldest winter design: -8°F (Howard County).
Typical equipment sizes in Des Moines
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Des Moines's design temperatures. Your house can need more or less; use the calculators for yours.
| House | Central AC | Gas furnace (95% AFUE) | Heat pump |
|---|---|---|---|
| 1,500 sq ft | 2 tons | 40,000 BTU | 2 tons + 8 kW backup |
| 2,000 sq ft | 2.5 tons | 60,000 BTU | 3 tons + 10 kW backup |
| 2,500 sq ft | 3 tons | 60,000 BTU | 3.5 tons + 15 kW backup |
AC size calculator Furnace size calculator Heat pump size calculator
Climate zone by county
From the 2021 International Energy Conservation Code (IECC), as published by the U.S. Department of Energy. The zone sets the insulation and window requirements in the energy code; the number runs from 1 (hottest) to 8 (coldest), and the letter is A for moist, B for dry and C for marine.
Zone 5A: cool, humid (84 counties, 89% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Adair County | 88°F | 0°F |
| Adams County | 88°F | 0°F |
| Allamakee County | 84°F | -6°F |
| Appanoose County | 90°F | 1°F |
| Audubon County | 90°F | -2°F |
| Benton County | 91°F | -1°F |
| Boone County | 88°F | -2°F |
| Bremer County | 89°F | -4°F |
| Buchanan County | 88°F | -4°F |
| Buena Vista County | 88°F | -4°F |
| Butler County | 89°F | -4°F |
| Calhoun County | 89°F | -4°F |
| Carroll County | 90°F | -2°F |
| Cass County | 90°F | 0°F |
| Cedar County | 90°F | 0°F |
| Cherokee County | 89°F | -4°F |
| Chickasaw County | 86°F | -6°F |
| Clarke County | 90°F | 1°F |
| Clayton County | 87°F | -3°F |
| Clinton County | 88°F | -2°F |
| Crawford County | 88°F | -2°F |
| Dallas County | 90°F | 0°F |
| Davis County | 89°F | 0°F |
| Decatur County | 89°F | 2°F |
| Delaware County | 88°F | -4°F |
| Des Moines County | 89°F | 2°F |
| Dubuque County | 86°F | -3°F |
| Fayette County | 86°F | -6°F |
| Floyd County | 86°F | -6°F |
| Franklin County | 88°F | -3°F |
| Fremont County | 91°F | 1°F |
| Greene County | 90°F | -1°F |
| Grundy County | 88°F | -4°F |
| Guthrie County | 90°F | -1°F |
| Hamilton County | 88°F | -4°F |
| Hardin County | 88°F | -3°F |
| Harrison County | 92°F | 0°F |
| Henry County | 90°F | 3°F |
| Howard County | 84°F | -8°F |
| Humboldt County | 89°F | -4°F |
| Ida County | 88°F | -4°F |
| Iowa County | 88°F | -4°F |
| Jackson County | 86°F | -3°F |
| Jasper County | 88°F | -2°F |
| Jefferson County | 90°F | 0°F |
| Johnson County | 90°F | 0°F |
| Jones County | 86°F | -3°F |
| Keokuk County | 90°F | 1°F |
| Lee County | 90°F | 3°F |
| Linn County | 88°F | -4°F |
| Louisa County | 90°F | 1°F |
| Lucas County | 90°F | 0°F |
| Madison County | 90°F | 0°F |
| Mahaska County | 90°F | 1°F |
| Marion County | 90°F | 0°F |
| Marshall County | 88°F | -4°F |
| Mills County | 92°F | 2°F |
| Monona County | 92°F | 0°F |
| Monroe County | 89°F | 0°F |
| Montgomery County | 90°F | 1°F |
| Muscatine County | 90°F | 1°F |
| Page County | 90°F | 0°F |
| Plymouth County | 91°F | -4°F |
| Pocahontas County | 88°F | -4°F |
| Polk County | 90°F | 0°F |
| Pottawattamie County | 90°F | 1°F |
| Poweshiek County | 89°F | 0°F |
| Ringgold County | 89°F | 2°F |
| Sac County | 88°F | -4°F |
| Scott County | 88°F | 0°F |
| Shelby County | 90°F | -2°F |
| Story County | 89°F | -2°F |
| Tama County | 88°F | -4°F |
| Taylor County | 90°F | 0°F |
| Union County | 88°F | 0°F |
| Van Buren County | 90°F | 0°F |
| Wapello County | 89°F | 0°F |
| Warren County | 90°F | 0°F |
| Washington County | 90°F | 0°F |
| Wayne County | 90°F | 0°F |
| Webster County | 89°F | -4°F |
| Winneshiek County | 84°F | -6°F |
| Woodbury County | 90°F | -1°F |
| Wright County | 88°F | -6°F |
Zone 6A: cold, humid (15 counties, 11% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Black Hawk County | 89°F | -4°F |
| Cerro Gordo County | 87°F | -7°F |
| Clay County | 90°F | -5°F |
| Dickinson County | 90°F | -5°F |
| Emmet County | 87°F | -7°F |
| Hancock County | 86°F | -6°F |
| Kossuth County | 88°F | -6°F |
| Lyon County | 86°F | -6°F |
| Mitchell County | 86°F | -6°F |
| O'Brien County | 88°F | -6°F |
| Osceola County | 88°F | -6°F |
| Palo Alto County | 87°F | -7°F |
| Sioux County | 88°F | -6°F |
| Winnebago County | 86°F | -6°F |
| Worth County | 87°F | -7°F |
Climate data as of 2026-10. Design temperatures may differ by a few degrees from the ACCA Manual J tables your contractor uses; winter values run a little warmer because recent winters have been milder.