I'm building a new shop/garage and am picking the garage doors right now (it's really more like a shop with some garage doors). There will be two single-car doors.

I'm looking at pre-insulated garage doors and am seeing R-values from 6.5 up to 18. Obviously "more is better" but is there a good way to calculate "how much better"? If I could find a way to compare heat loads that'd probably be best so I can see how much harder my A/C or heater will have to work. For reference I live in TN, so summers are 80's to 90's and very humid, and winters are around 30.