I don't know, but that doesn't keep me from speculating.
As you mentioned, ventilation is probably a major factor. It would be easy to check the high temp record at some Kentucky location, or even, more appropriately, average high temp in the normallly hottest month.
Then the question becomes: Does the temp on the upper levels, like my attic, climb 30 or more degrees higher than the outside air temp?
With lots of ventilation I'd guess the maximum temperature would be no more that 110 deg. F. With none at all I'd guess 140 deg. F.
However, the temperature of the bourbon inside the barrel won't have time to heat to the temperature of the surrounding air. Before it even comes close the air temperature will be dropping as evening approaches.
On second look the charming town of Loretto has an average high of 87 degrees in July, its hottest month. It's the same in Bardstown, Clermont and Louisville. In Lawrenceburg it's 86.
For you Tennessee whiskey fans it's 89 degrees F. in Tullahoma and Lynchburg.
Source for the above is AccuWeather.
In Bardstown the record highs range from 100 deg. F. to 105 over the months from June through September -- per weather.com.
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