I am building a house with 2x12 joists (I know...not sustainable but it's cheap and better if I use it before it becomes food for the pine beetle) and radiant heat.
At some areas the upper floor is exposed on its underside. I am going to use R40 fg batt insulation. What are your thoughts on whether the batts are tight to the underside of the radiant heated floor system or if I could gain more R-value if I allow a 2 inch space from the top of the batts to the underside of the plywood? My thoughts are the installed batts are creating a dead air space in the void.  Theoretically, that is what the batts are supposed to do too....

If I were you, I would use 2*12 I joists, cut a piece of reflective covered foam board to fit snugly between the webs and push it to where it is touching the top flanges, creating a 1.5" air space, and fill the other 10+ inches with fiberglass. You might get a bit less R-value, but you will be getting more of that heat you paid for directed up into your house. You can do the same thing with dimensional 2*12's but you will need to cut spacers by ripping pieces of 2*4 into small blocks.