“I would personally go with Tyvek”, “Plastic is OK for a short trip but gets pretty gnarly and messy after a few damp muddy nights. The silicon-nylon may not be durable enough to serve as a ground sheet and might puncture too easily.”
“One option…””…would be to buy a commercial tent footprint, roughly the size of your tarp shelter. They are generally made of polyurethane coated nylon (sort of a beefed up tent fabric) and are designed to take the abuse and keep moisture from getting through. They are relatively light, although not as light as Tyvek, and fold up easily into a small roll that can be stashed with your shelter or strapped on the outside of your pack.”
“I own the Zoid 1 tent and have the corresponding footprint. It weighs about 6-7 oz and rolls up small enough to stick in the mesh side pocket of my pack. Depending on the size of your shelter that may be all you'd need, however they do come in all sizes and shapes. REI sells them and they are available on-line from Campmor.”
(Mark (Toot), Atmlorphans mail list, Yahoogroups
)