I read something like this on a webpage describing how to insulate and sound deaden an aircraft.

A simple test will tell you if your panels are resonating. Tap on them, listen to see if it rings/resonates. Now put your hand on the opposite side of it where you Dynamat/ fatmat or whatever you are going to use will go.

Now tap on the panel again. If it is quieter, then Dynamat will help. If there is not much noise difference, you can probably skip that part, you do not need to do the entire interior surface to get great results.

Once you've deadend the panels that resonate, (typically large, open with little metal supporting them like roofs, floors, doors, firewalls, etc), cover the entire area with the insulator pad of your choice, being sure to seal the joints with aluminized tape to seal out heat, smell and sound.

You can put more Dynamat on to block sound etc, but it is expensive, and it's best use is to dead the resonation of the panels. Once the panels are deadend, blocking the rest of heat and sound over an entire suface like a floor or something, is more effectively done in terms of cost and performance with the insulator material from Aircraft Spruce. I bought mine in rolls.

I used the double sided this stuff for everything except the floors, excluding the transmission tunnel, where I used the thinner stuff. I taped the seams to seal it at those joints. Simply measure and ordered. The thick stuff will work on the floor excluding the transmission tunnel, but is really too thick to go anywhere else. The double foil thinner stuff is what you want. It comes it different widths.

I put it behind the door panels, behind the back seat, package tray, under the headliner(Dynamated first) and on the floors. I taped all the seams, so it is like a solid-custom fitted cover with no gaps for fumes, sound or heat to get through. In some cases, like where the sail panels go, I was not able to put anything other than Dynamat because of panel fitment issues.