Heres the run down.
When i had no money(hey i have no money now....) Anyways about a 2 years ago when i had my jeep I decided to sound deaden it some since it rattled bad with my Coaxials. I was only running 50w X 4 on kenwood coax's but it still hit hard enough to piss me off. So I did my research and found peel n seal. Went to lowes and picked up 4 rolls at $11 a piece.
I ended up doing all 4 doors. The Liftgate. And other random areas, under seats, trunk/cabin area and so on. Did it make a difference? Yes. It was practically night and day.
Mind you. I did not do much of the inner our outer door skins. I didnt cover up any of the large holes. The road noise was reduced by at least 75%. And my speakers sounded alot better.
In fact heres a picture of Peel n Seal on a door.
Now. Although it made a pretty big difference the car stunk for months of asphalt. Everytime it would get hot out it would smell some. Before I sold the jeep I noticed some of the peel n seal was peeling off as well. This is after sitting in the shade for 2 months. I never saw it melt in a puddle of asphalt but I refuse to use it in my car now. It's a black car with tinted windows and a dark interior. It gets scalding hot if it's in direct sunlight. I know peel n seal would easily melt.
My advice is if you want to add mass to your car. And want to do it cheaply. Use peel n seal on safe areas. What are safe areas? Areas where it doesn't matter if it melts theres no harm. IE The floor. Gravity cant get to it. The chances of it losing it's stickyness is low. If it melts it'll simply solidfy again where it was.
If you want true sound deadening on the doors. Go for Raamat or Secondskin Damplifier