The silver sheet will offer some insulation but not noise dampening, like you say.
I wouldn’t worry too much about sound insulation through the floor, as most noise comes in from the walls and roof when you are inside the vehicle.
If you will use the vehicle a lot in cold weather than definitely consider a more substantial floor insulation, but for spring/summer/autumn use the foil will be sufficient.
build you own campervan and live the van life