After installing the factory bitumen-based sound deadener on the floors, we continued with foil-backed sound absorber throughout the entire car, anywhere there is a mostly-flat panel. I like the foil-backed deadener because it accepts adhesive very well. Deadener without foil often has strange reactions to the solvents in adhesives, either going soft, sticky, or dissolving. The foil prevents this, and also adds a small amount of heat insulation as well.