What you use for a groundsheet under your shelter or under your sleeping bag depends on the ground surface.
If the ground is fairly smooth and you use a groundsheet mainly to keep your gear clean, the Gossamer Gear Ultralight Polycryo Ground Cloth is ideal because its very tough and very lightweight. No need to look any further.
For a bomber groundsheet, many hikers use conventional Tyvek housewrap, and there are places on the Web where you can purchase it piecemeal. It’s very durable, and will soften with a few trips through the wash machine, but it’s a bit on the heavy side.
A better option is Type 1443R Tyvek, which is a lighter, softer version used in Tyvek clothing. It’s inexpensive, weighs about the same as silnylon, and its waterproof like other Tyvek. It costs about $3.50 a yard (60 inches wide) at stores that sell kite-making supplies, like Into The Wind. Since it’s breathable, some people argue that it’s not waterproof, but I have yet to have a problem with water passing through it, even when camping on wet ground.
The breathability of this Tyvek makes it usable beyond groundsheets. An inexpensive Tyvek bivy works wonderfully for sleeping on the ground under the stars or inside a floorless shelter, and it adds a few degrees of warmth to your sleeping system. It’s very simple to construct a simple bivy by folding a sheet over lengthwise, sewing one end and one side, then turning it inside out. This yields a bivy with a 60-inch girth, which is a bit on the slender side. For a roomier bivy, especially if you put your sleeping pad inside the bivy, you will need to add a strip of Tyvek to the open side. Adding a 7 inch strip yields a 63-inch girth (the formula is to add 2 inches for each inch of girth you want to add, plus a 1 inch for seam allowances). Type 1443R Tyvek is very easy to sew, or you can tape it with filament or ripstop tape. If you want to seal the seams, use Roo Glue.
Post by Will Rietveld, Gossamer Gear Trail Ambassador