How many sq ft of sound deadner does one need?

[in_style]

I am doing the interior in my car, and I like my cra to be quiet. I have done some research here and it seems I can get away with 50 sq ft. But if I had 100 sq ft I could do the entire interior. But would that be too much deadner.
I know weight is a concern and 100 sq ft would beightin at 35 lbs, but to only do 1/2 the car makes no sense to me unless, it didn't have to be done.