Groundsheet

A groundsheet is a clean, waterproof surface on which to lay your sleeping pad, backpack and the rest of your kit. 

If you sleep in a tent, this may be covered, but it is a good idea to have a stand-alone groundsheet for when you dons't bring the tent.

Tyvek Groundsheet

Tyvek is used in construction and commonly called "house wrap" by the linear foot. The fabric is 9 ft. wide. It is a popular choice for use as tarp or tent footprints and groundsheets. It is manufactured by the DuPont Corporation, and the manufacturer states that it maintains comfort by keeping out hot or cold air, that it protects against water damage, and that it breathes, letting harmful moisture escape. Tyvek is very stiff and crinkly at first, and should be folded repeatedly to diminish crinkling. BUY: Buy Tyvek