Geoff,

Welcome!

Yes, you are correct that you will need A/C. I am in North Mississippi and I understand the 85 degrees and 85% relative humidity at 4:30 am. It is disgusting! I mean it is too hot to go fishing! Anyway, I have not started my build (heck I am still waiting on the response to my offer on some land), but I designed A/C systems and sold custom systems in my younger days. Thermal mass will not be as much of an asset for us as the ones up north or in the mountains. They have it good in that by the time the day's summer heat makes it half way through the logs during the day, night time arrives and it is cold (or cooler) outside and thus, the logs start radiating heat back to the outdoors. Plus the outside of the log will be 60 degrees or less on the outside in the early am. However, if it is 90 degrees at midnight and 85 degrees at 4:30 am you can count on your exterior walls being 85 degrees. (I don't know about others, but when it is 85 degrees and high humidity, I want me some A/C.) Thus, you will have heat transfer from the outside to the inside. The good news is that a good designer will take into account the design conditions where you live and insulation value of your walls (thicker walls have more insulation value) to come up with a properly sized system. I would strongly recommend wrap around porches both to keep the water and sun off your walls. Solar load can be intense down south and even with thick walls the heat will eventually get to the inside.