In this Instructable, I'll show you how to make a cheap bivy that works well in decent weather and is not terribly heavy. In rainy conditions, you should have additional shelter over you. (Commercial Bivys start around $80.)
This bivy is constructed from 14'-15' of 3' wide Tyvek House Wrap, and double-sided carpet tape. The basic design is a full length of tyvek folded at the foot and sealed on the sides.
One important note: This bivy does not breathe. Never seal up an unvented bivy over your head!
Step 1: Materials/Tools
Approximately 15' of 3 foot-wide Dupont Tyvek Home Wrap (or similar).
Double-Sided Carpet Tape (the permanent kind)
Tyvek Tape (optional)
Wallpaper seam roller (Optional)
Build Time: Approximately 2 hours, with a break for drying the tyvek...
Step 2: Step 1: Measure and Cut
Fold the end, and roll out the top sheet, stopping short of the full length.
You want the bottom to be longer than the top so your head pokes out the top, but under your head is protected by the bivy.