All year
What a Mixed-Humid Climate Means
A mixed-humid climate, as DOE defines it, gets more than 20 inches of rain a year, about 5,400 heating degree days or fewer, and winter months averaging below 45°F, and every Kentucky county fits it.
What does mixed-humid mean?
Mixed-humid is DOE's label for places that need real heating in winter and real cooling and dehumidifying in summer. DOE Building America's Guide to Determining Climate Regions by County (August 2015) sets three tests: more than 20 inches of precipitation a year, approximately 5,400 heating degree days (65°F base) or fewer, and an average monthly outdoor temperature that drops below 45°F in winter.
The same guide says the mixed-humid zone covers the parts of IECC climate zones 4 and 3 in moisture category A that sit above the warm-humid line. The letter is the IECC moisture category. Past 5,400 heating degree days, the guide's cold climate begins, spanning 5,400 to 9,000 heating degree days and matching IECC zones 5 and 6.
A heating degree day counts how far a day's average temperature falls below 65°F. A day averaging 35°F adds 30. Add a year of them and you have a single number for how much heating a place needs.
Which climate zone is your county in?
If you live in Kentucky, your county is Mixed-Humid 4A on the 2015 Building America county map from DOE: all 120 Kentucky counties carry that label in the guide's table, from Jefferson and Fayette to Pike and McCracken. The neighboring states split.
| State | Mixed-Humid 4A counties | Other zones | Examples |
|---|---|---|---|
| Kentucky | All 120 | None | Jefferson, Fayette, Kenton: 4A |
| Ohio | 9 | 79 Cold 5A | Hamilton 4A; Franklin, Montgomery, Butler 5A |
| Indiana | 30 | 62 Cold 5A | Clark, Floyd, Vanderburgh 4A; Marion 5A |
| West Virginia | 27 | 28 Cold 5A | Kanawha, Cabell 4A; Monongalia, Raleigh 5A |
| Tennessee | 81 | 14 Mixed-Humid 3A | Davidson 4A |
Ohio's nine 4A counties are Adams, Brown, Clermont, Gallia, Hamilton, Lawrence, Pike, Scioto and Washington, a band along the river. Later editions of the IECC moved some U.S. counties to warmer zones, and this site has not checked a primary county list for those editions, so treat the table as the 2015 map.
Zones feed into building codes. DOE's code status page for Kentucky lists the 2009 IECC as the state's residential energy code, effective October 1, 2014, and Ohio's page lists the 2018 IECC with amendments, effective July 1, 2019.
How do heating and cooling needs compare here?
Heating outweighs cooling at every major station in the region. In NOAA's 1991–2020 normals, Louisville averages 3,935.6 heating degree days and 1,742.2 cooling degree days a year, both on a 65°F base, from the annual normals file for Louisville International Airport.
| City (station) | Heating degree days | Cooling degree days | Days at 90°F or hotter |
|---|---|---|---|
| Nashville (airport) | 3,364.0 | 1,873.0 | 54.1 |
| Louisville (airport) | 3,935.6 | 1,742.2 | 42.7 |
| Evansville (airport) | 4,339.7 | 1,524.9 | 41.0 |
| Charleston WV (Yeager) | 4,353.2 | 1,162.2 | 23.6 |
| Lexington (Blue Grass) | 4,432.4 | 1,289.0 | 25.1 |
| Cincinnati (CVG) | 4,892.3 | 1,159.6 | 22.1 |
Every station in that table sits under the 5,400 heating-degree-day line, Cincinnati's airport the closest at 4,892.3. That is the arithmetic behind the mixed-humid label. Nashville's and Evansville's normals rest on 25 and 28 years of data rather than a full 30, which NCEI still rates as standard.
The cooling side matters more than the numbers suggest, because summer load here includes moisture, not just heat. Those cooling degree days come with sticky dew points, covered below and on the humidity control page.
How cold and how humid does it get, city by city?
Winters hover around freezing and summers run muggy, with real cold arriving only in short bursts. NOAA's 1991–2020 monthly normals give Louisville a January mean daily low of 27.8°F, and Cincinnati's airport 23.1°F, the coldest of the major stations here.
Below-zero nights are rare. In the annual normals, Louisville averages 0.5 nights a year at or below 0°F, Lexington 0.9 and Cincinnati's airport 2.5, against 74.0, 89.9 and 99.8 nights at or below 32°F. Records show what the tail can do: Louisville's all-time low is -22°F on January 19, 1994, per the National Weather Service, and Winter Storm Elliott took Louisville to -5°F on December 23, 2022.
Summer humidity shows in the dew point. In NOAA's hourly normals, a normal July 15 at 3 p.m. in Louisville is 86.7°F with a 66.9°F dew point, and one July 15 afternoon in ten reaches a 73.9°F dew point or higher at that hour. Averaged from NOAA hourly normals, Louisville's July dew point comes to 66.7°F, Cincinnati's 65.2°F and Nashville's 68.0°F.
Winters hover around freezing and summers run muggy, with real cold arriving only in short bursts.
What does the climate mean for a heat pump?
A mixed-humid climate is the kind of place heat pumps were built for. DOE's Federal Energy Management Program purchasing guidance says heat pumps work well in hot-dry and mixed climates, while cold climates need specially designed units that run at lower outdoor temperatures before switching to resistance heat.
Four things follow for an Ohio Valley house:
- Winter is mostly frost weather, not deep cold. Outdoor coils frost from roughly the mid 40s down to the high teens, ORNL found, which is the heart of a Kentucky January. Expect regular defrost cycles.
- Backup heat is for short spells. Standard units lose output in the low to mid 30s, and the few single-digit mornings are when the balance point and backup heat matter. Whether a cold-climate model earns its price depends on how often your county gets that cold.
- Size for the climate, not the square footage. PNNL's Building America Solution Center gives heat pumps in IECC zones 4 through 8 a cooling-capacity band of 95% to 125% of the design heat gain. Louisville's winter design temperatures are 16°F at 99.0% and 10°F at 99.6% in the engineering weather data NOAA hosts from the Air Force (period 1988–2017); the Manual J your contractor runs may use other values.
- Summer is a humidity job. EPA's mold and moisture guide advises keeping indoor humidity below 60%, ideally 30% to 50%, which a heat pump does as it cools when it is sized right.
Loose ends
Smaller questions readers ask about this subject, answered briefly.
Is Kentucky a cold climate for heat pumps?
Not by DOE's definition. Every Kentucky county is Mixed-Humid 4A on DOE Building America's 2015 map, and the major stations average well under the 5,400 heating degree days where the cold zone starts.
Is Cincinnati in a different climate zone than Kentucky?
Hamilton County, Ohio, is 4A on the 2015 map, like Kentucky. Most of Ohio, including Franklin and Montgomery counties, is Cold 5A.
What does the A in 4A mean?
It is the IECC moisture category for the zone. DOE's guide places mixed-humid in the category A portions of zones 4 and 3.
How often does Louisville drop below zero?
About once every two years on average: NOAA's 1991–2020 normals give 0.5 nights a year at or below 0°F at Louisville International Airport.
Why do summers feel worse than the temperature says?
Dew point. A normal July afternoon in Louisville pairs about 87°F with a dew point near 67°F, and one such afternoon in ten brings a dew point of 73°F or more.
Does the climate zone change which heat pumps you can buy?
Federal minimums for heat pumps are national, with no regional split. The zone matters more for sizing, duct and insulation choices, and whether a cold-climate model is worth it.