A bivouac sack (also known as a bivy sack, bivi bag or just bivy) is an extremely lightweight, waterproof alternative to traditional tent systems. It is used by climbers, mountaineers, hikers, ultralight backpackers, soldiers and minimalist campers."
Bivy sacks keep you warm while making a light weight waterproof covering for your sleeping bag.
Oh and this is in the stay warm contest so vote for me!
Step 1: Materials
What you need:
~3 yards of Gortex
~3 yards of Black taffeta
Step 2: Step 1
Cut both fabrics to 2.5 yards by 40 inches.
pair up the fabrics with the white side of the gortex faceing out. Sew 1/2" away from the edges.
Step 3: Step 2
Turn the whole thing right-side-out and sew in some zippers. I used a 30" and a 8" zipper.
You could also use velcro but I hate it when things get stuck in it.