The way they insulate small airplanes and the way I have done two of my cars is to use a Dynamat type butyl self adhesive product to stop the panel from resonating (you don't have to cover the whole panel, just enough to stop the resonnation of the panel), and then they use a double foil sided mat with non-flamable stuffing, between the two products.
It is much more effective when you use both products. The foil mat can be taped together with that aluminum duct tape with the peel off wax paper. It makes a vapor, sound and heat barrier that compliments the Dynamat type material.
I found that Dynamat Extreme can be purches at nearly half off with free shipping with a little searching on-line. The foil blanket material is available from either Aircraft Spruce or Speedway Motors.
My hardtop E-Body sounds like your inside a vault when you close the door. It's the quietest Mopar I've ever been in on the highway.