I decided to go more for sound dampening than the sound deadening route. Most if not all of the commercial dampening material has butyl tape for its foundation with aluminum foil attached. They may put a layer of 3/16" closed cell foam or similar over that.
I opted to purchase this 20 mil butly tape instead of a brand name product since it was way less money. Bought a case of 12 rolls for about 1/2 of what it would have cost me to do 1 side with the brand name products.
Put 2 layers on the firewall and front floor areas and 3 layers on the tunnel hump area and only used 2 1/2 rolls so I have plenty left for doors and rear quarter areas.