My barn has 3 12' x 12' roll up doors - not barn type like yours, but I think my efforts may be applicable. On my doors, along the top, there was a 2" gap along the entire width of the door, which in my fuzzy math equals a 2' hole for each door. A 6' opening defeats a lot of the reasons I had the barn insulated.
To fix, I installed brushes along the entire width on the header of each door. It cost less than $200 total and the result is pretty amazing. Even though the doors are not insulated and there are gaps along the edges and rails, it knocked a huge amount of draft down. With a simple ceramic heater going the barn holds a great deal of warmth.
http://www.memtechbrush.com/brush-seals/sectional-overhead-doors.htm
These are easy to install, and give some flexibility on installation. Took a couple of hours, but very well worth it.