[Zeke]

I've done this. You're right in that if the air flow has no exit, it's not going to help as much as mass. I did this with rigid foam. Wires went up first, then the lights and finally the foam.

We had more beams; less spacing, so center nailers were not necessary. What I liked was that we held the finished ceiling up a bit so that the original beams still showed. That breaks up the ceiling for a nice look, especially painted somewhat differently.

That's what I'd do with yours. I'd box the existing so they were deeper and fill in between with 3" foam and your T & G. You will need nailers at the beams and one down the center.