wy0mn-
Just a minor technicality, but just because you're burying them in the ground doesn't mean your heating/cooling troubles are solved with "free insulation". Maybe you meant something else, and/or maybe I am misunderstanding or not explaining my thoughts clearly, but all burying your boxes in the ground does is allows your containers to be more close to the annual average ground temperature (In Tennessee, that might be maybe 55-60 degrees?) In winter, you need to heat your box from 55 degrees up to 70 degrees. In summer, you need to heat your box from maybe 60 degrees, up to 70 degrees. And no, drawing warm outside air probably won't work in the summer, doe to humidity and condensation. Look on the outside of your iced tea glass. Now envision that being the walls of your home. Certainly only needing to heat your inside air from 55 degrees to 70 degrees is better than whatever temperature it might be outside in the dead of winter, but having to do it 365 days a year isn't free.
If I'm nitpicking on something you understand and just chose the wrong words for, I apologize. If I'm not stating my case properly, again I apologize. Just the words "free insulation" threw up a red flag for me. That's all.
Good luck!
John