[bcrdukes]

Ventilation / air flow is likely the culprit.

In my parents' house, the room on the south east side was the absolute hottest, while other rooms throughout the house would no more than what is measured or reported outside. The delta between outside temp and the temperature inside that one specific room was almost always 12 degrees hotter and stuffier.

We found out that the house's previous owner moved the dog from that one room because it nearly died from heat exhaustion to the main floor and basement because of poor ventilation.