Tyvek(TM) bivvy/over bag instructions

Total weight 320 g

This just made sense to me. I have a synthetic barrel bag rated to -15 C which was good in a draughty log cabin on a -20 C night even after the stove went out, but was cold on a -10 C night at 100% relative humidity with only myself in a large 2-person tent. If I get serious about winter, I'll buy the lightest, cheapest down mummy summer bag I can get and use it inside the synthetic bag with the Tyvek bivvy as an overbag.

I was making a tarp at the same time, so I had "improved" the Tyvek by the traditional technique of screwing it up by hand, giving it a cold wash cycle and a spin, then a hot wash cycle and spin. This does not seem to be mandatory if you're just making the bag, and must weaken the waterproofness a fair bit. If your favourite camping partner is a light sleeper, however, crisp new Tyvek could be an issue.

Materials: Tyvek and sheathing tape. I made mine with a 6 ft. strip of 9 ft. wide Tyvek, but of course it depends on your bag. You need enough to match the outside of your bag with about 4 in. extra at the foot and sides and 2 in. extra at the head. See sketch 1.

1. Cut Tyvek with 4 in extra at sides and foot and 2 in. extra at head including both hood and front of neck. See sketch 1 for details at head end.

2. Fold lengthwise (printing outside) with 4 in. overlap and tape along length to ends. This should give you an extra 4 in. circumference compared to your sleeping bag, preserving the loft.

3. Turn inside out so printing is inside. Tape lengthwise seam to ends.

4. Seal foot end by folding tape over ends of fabric, in other words, without a hem. To allow you to focus on getting a good seal, do 4 in. at each corner first with tape extending 1 in. past sides of bag. Don't worry about excess tape. Seal rest of foot end with tape.

5. Fold over 4 in. of foot. Fold corners of hem at 45 degrees, tape corners down, then tape down rest of foot hem.

6. Turn bag right-side in, (printing outside). Push foot end out cleanly, it should look like the inside of a paper bag. Find the seam, push out the corners well, and reinforce by taping the outside. Spend a bit of time getting clear about where you want to tape this before committing yourself, and it will work.

7. Cut darts 2 in. deep around the hood seam. Fold over towards the outside (or back, if that makes more sense to you) and tape down one at a time. Make sure you leave nice 1 in. open ends for the drawstring. It will be easier to thread the drawstring if you start at one end and work towards the other. Tape over the exposed cut edges of fabric, with an extra inch at each end (fold over the outer end).

8. Fold over the neck seam to the outside (or front). Tape down, leaving nice 1" open ends.

9. From the scraps, cut a 3 in. wide rectangular strip about 3" shorter than the widest part of the bag at the shoulders. Tape across the back level with the neck seam.

10. I had a bunch of vulnerable-looking fabric edges at the open end of the bag, so I did my best to reinforce where appropriate using tape. Somehow I managed to tape over the end of one of the drawstring seams. If you "improve" your Tyvek, it will not appreciate you trying to remove tape. If you make the same mistake as I did, it's better to carefully slit the tape.

11. Thread drawstring Save a bight at the middle of the string, and pass one end across the front of the neck and back through the hood. Pass the other end across the front of the neck and back behind the neck. See sketch 2. If you pass the bitter ends through the bight in opposite directions and then join them with an doubled overhand you won't lose the ends.

Thus completeth the bag. If it's cold enough or wet enough to use it, the lack of a zipper will be a good thing. Tyvek is so slippery you can almost toss the foot of your sleeping bag into it and it will slide all the way it. You get in as if you were putting on a sock - legs first, then get all of the loose material over your hips, then sit up and pull it up to your shoulders. I find it easiest to tighten the hood on my sleeping bag about halfway first, then take care of the overbag hood and collar, then finish the sleeping bag. If you want to be sure the overbag stays tight, tie a slippery overhand tight to the bight in the drawstring.

Test your bag. I can take about 5 minutes of my sleeping bag indoors. With the overbag it gets intolerable in about 30 seconds. This is where my partner walks in and collapses in hysterical laughter at the sight of the giant white maggot on the living room floor. I took the overbag alone to the back yard and rolled around in the wettest part and stayed dry. As with any waterproof-breathable material I'm sure keeping it clean is important.