What Home Depot insulation have you used under your carpet?

The insulation that's mostly air with foil on the outside won't do anything to prevent sound, it's thermal insulation.

If you want to quiet down the car you want dense foam type insulation. The kind I got has foil on 1 side.

The kind linked in this thread is just pure thermal insulation, the description doesn't even mention noise. I guess if you're afraid of heat/cold going through the floorboards it would be useful, but other than that it's not doing anything for you. The main concern should be sound, in which that insulation won't help at all.