We have 11 ft ceilings in our 1 story house in Dallas (current temp: 105). My friend says that higher ceilings result in less A/C usage during the summer compared to 8 ft ceilings (assuming that everything else is the same). His theory is that with high ceilings, the hot air stays up top where you can't feel it (and where it can't trip the t-stat). So the A/C only runs to cool the bottom 6-8 ft of air in the house and the air above that stays warmer. I understand what he's saying, but I think the air is going to mix quite a bit, so while there might be a small temperature difference between the bottom and the top, its not going to be anything substantial (maybe a degree or two). And with higher ceilings, you have a larger air mass to cool, which means more A/C time.
So the question is, if you had two identical houses next to each other with the same insulation, shade, windows, t-stat setting, and A/C system, but one house had higher ceilings, which one would use the A/C more?