The 1/8" thick version. Dense fiberglass mat with thin aluminum foil layer, sticky back. I found it to be lightweight, inexpensive, effective, and available in stock at most big box home improvement stores. In my opinion only good for under carpet only, nowhere else...adhesive isn't good enough for vertical application to insides of doors, etc. It'll fall off. But under carpet? It can't fall off if it's already laying on the floor and held down by the carpet...2 rolls covered the majority of my floor for less than $20 each roll.
It's not light...basically a solid tar/asphalt type of product with thick aluminum foil on one side. This is good stuff for lining insides of doors and quarter panels, etc. If applied to a clean surface, it'll stick seriously and forever. It's dense and heavy. Each roll is like 10 to 15 pounds I think. It'll add some weight to your car.
Read the reviews on that link...it seems there are a whole lot more people using this product for soundproofing/insulating their car than for actually fixing their roofs & gutters...
