Kentucky climate zones and HVAC design temperatures
All of Kentucky is in IECC climate zone 4A (mixed, humid). In Louisville, heating and cooling equipment is sized for 91°F in summer and 16°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Louisville | 4A | 91°F | 16°F | 61°F |
| Lexington | 4A | 90°F | 15°F | 60°F |
| Bowling Green | 4A | 92°F | 18°F | 62°F |
| Owensboro | 4A | 91°F | 15°F | 61°F |
| Paducah | 4A | 92°F | 17°F | 62°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 (Allen County); coldest winter design: 13°F (Boone County).
Typical equipment sizes in Louisville
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Louisville'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 | 2.5 tons + 5 kW backup |
| 2,000 sq ft | 2.5 tons | 40,000 BTU | 3 tons + 5 kW backup |
| 2,500 sq ft | 3 tons | 60,000 BTU | 3.5 tons + 5 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 4A: mixed, humid (120 counties, 100% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Adair County | 89°F | 15°F |
| Allen County | 92°F | 18°F |
| Anderson County | 90°F | 14°F |
| Ballard County | 92°F | 17°F |
| Barren County | 90°F | 16°F |
| Bath County | 88°F | 16°F |
| Bell County | 89°F | 19°F |
| Boone County | 89°F | 13°F |
| Bourbon County | 88°F | 16°F |
| Boyd County | 89°F | 15°F |
| Boyle County | 90°F | 14°F |
| Bracken County | 89°F | 15°F |
| Breathitt County | 87°F | 16°F |
| Breckinridge County | 89°F | 14°F |
| Bullitt County | 92°F | 17°F |
| Butler County | 92°F | 18°F |
| Caldwell County | 91°F | 17°F |
| Calloway County | 91°F | 18°F |
| Campbell County | 89°F | 14°F |
| Carlisle County | 92°F | 17°F |
| Carroll County | 90°F | 14°F |
| Carter County | 89°F | 15°F |
| Casey County | 90°F | 14°F |
| Christian County | 91°F | 17°F |
| Clark County | 88°F | 16°F |
| Clay County | 88°F | 17°F |
| Clinton County | 89°F | 15°F |
| Crittenden County | 91°F | 15°F |
| Cumberland County | 89°F | 15°F |
| Daviess County | 91°F | 15°F |
| Edmonson County | 92°F | 18°F |
| Elliott County | 89°F | 15°F |
| Estill County | 90°F | 16°F |
| Fayette County | 90°F | 15°F |
| Fleming County | 89°F | 15°F |
| Floyd County | 86°F | 17°F |
| Franklin County | 90°F | 14°F |
| Fulton County | 91°F | 19°F |
| Gallatin County | 89°F | 13°F |
| Garrard County | 90°F | 14°F |
| Grant County | 89°F | 13°F |
| Graves County | 91°F | 18°F |
| Grayson County | 89°F | 14°F |
| Green County | 90°F | 16°F |
| Greenup County | 89°F | 15°F |
| Hancock County | 91°F | 15°F |
| Hardin County | 89°F | 14°F |
| Harlan County | 88°F | 19°F |
| Harrison County | 88°F | 16°F |
| Hart County | 90°F | 16°F |
| Henderson County | 90°F | 14°F |
| Henry County | 90°F | 14°F |
| Hickman County | 91°F | 19°F |
| Hopkins County | 90°F | 14°F |
| Jackson County | 88°F | 17°F |
| Jefferson County | 91°F | 16°F |
| Jessamine County | 90°F | 15°F |
| Johnson County | 88°F | 17°F |
| Kenton County | 89°F | 13°F |
| Knott County | 86°F | 17°F |
| Knox County | 88°F | 17°F |
| Larue County | 89°F | 14°F |
| Laurel County | 88°F | 17°F |
| Lawrence County | 89°F | 15°F |
| Lee County | 87°F | 16°F |
| Leslie County | 88°F | 19°F |
| Letcher County | 82°F | 16°F |
| Lewis County | 89°F | 15°F |
| Lincoln County | 90°F | 14°F |
| Livingston County | 91°F | 15°F |
| Logan County | 92°F | 18°F |
| Lyon County | 91°F | 18°F |
| Madison County | 90°F | 16°F |
| Magoffin County | 87°F | 16°F |
| Marion County | 90°F | 14°F |
| Marshall County | 91°F | 18°F |
| Martin County | 88°F | 17°F |
| Mason County | 89°F | 15°F |
| McCracken County | 92°F | 17°F |
| McCreary County | 89°F | 15°F |
| McLean County | 91°F | 15°F |
| Meade County | 89°F | 14°F |
| Menifee County | 88°F | 16°F |
| Mercer County | 90°F | 14°F |
| Metcalfe County | 90°F | 16°F |
| Monroe County | 90°F | 16°F |
| Montgomery County | 88°F | 16°F |
| Morgan County | 87°F | 16°F |
| Muhlenberg County | 91°F | 15°F |
| Nelson County | 89°F | 14°F |
| Nicholas County | 88°F | 16°F |
| Ohio County | 91°F | 15°F |
| Oldham County | 91°F | 16°F |
| Owen County | 90°F | 14°F |
| Owsley County | 87°F | 16°F |
| Pendleton County | 89°F | 14°F |
| Perry County | 87°F | 16°F |
| Pike County | 86°F | 17°F |
| Powell County | 88°F | 16°F |
| Pulaski County | 88°F | 18°F |
| Robertson County | 89°F | 15°F |
| Rockcastle County | 90°F | 16°F |
| Rowan County | 89°F | 15°F |
| Russell County | 89°F | 15°F |
| Scott County | 90°F | 15°F |
| Shelby County | 90°F | 14°F |
| Simpson County | 92°F | 18°F |
| Spencer County | 91°F | 16°F |
| Taylor County | 90°F | 14°F |
| Todd County | 91°F | 18°F |
| Trigg County | 91°F | 17°F |
| Trimble County | 91°F | 16°F |
| Union County | 90°F | 14°F |
| Warren County | 92°F | 18°F |
| Washington County | 90°F | 14°F |
| Wayne County | 89°F | 15°F |
| Webster County | 90°F | 14°F |
| Whitley County | 88°F | 17°F |
| Wolfe County | 87°F | 16°F |
| Woodford County | 90°F | 15°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.