I see know reason for any insulation between the old floor and ply tongue and groove ¾ if you well be placing laminated floor down since it should have plastic placed down before the laminated floor is put down. But have you made sure you have the framing to hold up a second floor. Was it storage for hay and such? What about the planks that are down now, spacing and condition could they be sanded down and stained, how thick are they? City boy here so dont know much about barns.
If it's a standard issue barn, that is originally used for livestock and hay, it should be more than stong enough to hold a second floor remodel. I grew up on a farm, Those first forks of bales would drop from the track and slam down on the floor. If they can take that ........ and take stacking tons of bales to the ceiling they can handle most anything. That is if the structure is still sound.