Some of the bubblewrap stuff is rated differently than say fiberglass or foam...if it has an aluminized layer, they treat it as a reflective layer in addition to the insulating/conduction layer. Heat moves in both radiation (black body) and reflective manner. Fiberglass and say foam slow heat migration by minimizing radiation losses. Foil barriers don't slow it, but redirect most of it. A radiant barrier needs an air gap on each side in order to work or it becomes a good conductor.
So, if the cold side of the floor is unheated, you need to slow radiation into that cold space, you need something like fiberglass or foam (it would be tough to pack cellulose into that space). Spray foam might mean any maintenace repairs on pipes and wiring would be nearly impossible, so foam sheets or fiberglass are probably best. Per inch, foam has the highest insulating value. If you can afford to lose some headroom, you might consider some of the foil faced foam sheets mounted to the bottom of the joists and gain both radiation and reflective benefits.
See what the others have to say. Adding fiberglass is doable, but can get messy with a lot of pipes and wiring to get good coverage. You will likely have to split the batts and put part above and below some things. If you compress it too much, you lose a lot of its insulating capabilities.