Tyvek seems like a terrific material to make a lightweight bivy sack.
We thought that Tyvek would be awesome for making our own bivy sacks. It seems like the ideal material: lightweight, strong, waterproof, and breathable. So, we decided to make a basic bivy for both of us.
We used two sheets of Tyvek to make the equivalent of a top sheet and bottom sheet. Step number two was to lay the sheets out, one on top of the other. Using Tyvek tape taped around the bottom 3/4 of the two sheets. The net effect was something like a giant Tyvek sleeping bag into which our actual sleeping bags would fit. We also used a soldering iron to melt some holes into which we taped grommets. Our thinking was that we might want to use some line to tie open the end of the bivy.
Our first night was in the snow at about 5°F. It was very calm, but very colde. I kept my head at the open end of the bivy. Seems kind of obvious to me. I tied open the end and could feel some of that really cold air on my face so I slid down in my bag a little and ended up being comfortable through the night. My down bag stayed dry and warm.
My buddy seemed to think it would be a good idea to go in head first and he didn’t think it wise to tie open the end of his bivy. His breath ended up condensing inside the bivy over the course of the night. In the morning he said that every time he rolled over there was a little mini-shower of ice crystals. It was not so pleasant.
Because he was freezing, we went far down in altitude for the next night with the Tyvek. We ended up actually car-camping. It so happens that the second night was a windy night even though it was much warmer. In the morning we agreed that being in the Tyvek was like sleeping inside a potato chip bag. It was quite loud!
On the up-side, a woman from a nearby campsite asked us if we were testing equipment for a technical manufacturer. She used to work for Patagonia and said she saw a lot of people testing out unusual equipment. Apparently we at least looked cool enough that she thought we weren’t just nut jobs!