How should I insulate hydronic radiant heating under floorboards
theory is that the pipes are hot enough that radiation is signficant.  Putting a low  IR emmisivity surface reduces the amount of radiant energy the bottom of the cavity absorbs.

If you just have a low e foil, then you cut the radiation in half.  However by conduction the entire air space gets warm.  So you will have the foil as a barrier for conduction too.  It's not very good at that.

Me, I would put friction fit paper backed bats in. leave a 2" thick air space at the top of the cavity, so that you have fewer hot/cold spots on the floor above.