I didn't mean to jump on you, but pro-asphalt posts worry me //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif Top layers aren't strictly necessary, but they do protect the interior from adhesive at the seam, are very good for gasketing trim panels to reduce buzzing and may help reduce reflections. Any closed cell foam - Ensolite MLV, Second Skin Overkill, eDead's V4, Neoprene or pretty much anything that doesn't smell bad should be fine. If your goal is to stop noise intrusion into the passenger compartment look into one of mass loaded vinyl/foam composites available from a variety of sources. Barriers on top of a vibration damper are the best way to fight noise. I don't suggest gluing foam or barriers down on horizontal surfaces - just makes it harder to get back in there if you need to and gravity and your carpet or trim will do the job.