www.dynamat.com
They have really good products, I don't know about the light weight part but they have sound dampening and insulation...
www.cascadeaudio.com
They have VERY good products, again, don't know about the light weight stuff, but the db levels drop.
Both of these products are sheet materials, Dynamat has liquide materials, but I don't know how well they work, if you want liquide (brush, roll, or spray on) material, here's a link:
www.stealthkote.com
They work wonderfull, and a gallon of it weighs in at only 13.2 lbs.
Not too bad, but what can you do when you want peice and quite.
Well, Super, I hope that help you out some... let me know what you think!
John