I taped up the black plastic on the under carriage with black duct tape.
I used cellulose insulation in the floor since I had to shove it several feet rearward due to all the wet fiberglass batt I had to remove.
Tip: be sure to staple the plastic to the floor studs from underneath about every 4" or so. That way, you won't be over-stuffing the void underneath.
Tip: use a strong drill and thinset mixer paddle to break up the cellulose. Put some in a plastic garbage can and mix away.