A Bivy, or Bivouac Sack is a bag for your sleeping bag. In good weather, they keep your sleeping bag clean, in bad weather, they keep you dry (mostly)... Bivys are basically a waterproof shell that protects your sleeping bag. They have an added benefit of keeping dew off your sleeping bag, and slightly increasing your warmth. The only down-side is that the bag will trap moisture (perspiration) in the bag. In hot/humid conditions, this can be a big problem.

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

Required Materials:
Approximately 15' of 3 foot-wide Dupont Tyvek Home Wrap (or similar).
Double-Sided Carpet Tape (the permanent kind)
Velcro
Tyvek Tape (optional)

Tools:
Scissors
Measuring Tape
Wallpaper seam roller (Optional)
washing machine

Build Time: Approximately 2 hours, with a break for drying the tyvek...

Step 2: Step 1: Measure and Cut

Lay out your tyvek, Printing side down. Roll out enough to cover the length of your sleeping bag plus a foot or two.

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.

Step 3: Tape Away!

Time to seal those edges.

Before you do, make a note of which side the sleeping bag's zipper is on. This is important, as we won't be fully sealing the bivy.

With the sleeping bag facing DOWN, the side without the zipper will be fully sealed. Start on that side.

Mark the location of the top sheet on the bottom, so you'll know where to stop taping. Plan on leaving about 1inch untaped.

Carefully tuck the tape into the corner, and run the tape up to the mark. Be careful as this tape is sticky stuff. It's not a bad idea to leave a very small gap between the tape and the edge to avoid getting the tape on your work surface.

Step 4: Seal the Seam

Starting at the corner, working in 6" increments, remove the backing from the tape, and carefully attach the top sheet.

Continue working until the seam is complete.

Be sure to press firmly on the seam to ensure good adhesion. I used a wallpaper seam roller (optional).

Step 5: Fold the Corner

stick a small square of tape in the corner. Fold the corner over. This will improve both the look of the finished bag, as well as ensure a good corner seal.

Step 6: Seam the Opposite Side

When we invert the bag, the unsealed side will line up with the zipper on your sleeping bag.

For this side, we're only going to seal about 1/2 the seam.

Using a sharpie, mark the end-point, and tape as in step 4.

Peel and stick the top sheet, and fold the corner as in steps 4 and 5.

Step 7: Crumple Time

If you haven't noticed, the Tyvek is stiff and really really loud. A quick run in the washing machine will fix that... But first, we need to crumple it to make it easier to handle.

First, reach into the bag and pull the foot through the top, inverting the bag. This leaves a nice looking seam, and the printing on the inside of the bag.

Next crumple the bag as much as you can and stretch it out. Do this a few times. This wrinkling makes the tyvek much more pliable, and easier to get in the washing machine..

<EDIT> December 2015: It has been suggested (below) to pre-crumple your material before making... That way you don't end up with a 200lb bag of water in your washing machine... The advantage of post-crumpling is easy assembly with nice flat tyvek, but it would be easier to crumple the material first... I'll let you decide...

Step 8: Into the Washer

Fill a top-loading machine with cold water. DO NOT ADD DETERGENT! Detergents have been reported to break down the tyvek, and they're not necessary.

Push the bag into the washing machine, and start on a regular cycle.

At first, the tyvek won't move much, and you may have to push it into the machine.

As the machine works, the material will be more pliable. When the wash cycle is nearly complete, the material should be moving freely in the water. If it is not, stop the machine and remove the tyvek. You may find that only some of it has been softened. Re-load the tyvek, re-set the timer, and let it continue.

Important note: DO NOT LET THE MACHINE SPIN! (Mine crawled about 2 feet before I could stop it!) The tyvek bag will hold water, and the spin cycle could tear the bag or cause damage to your machine.

When the tyvek moves freely in the washer (see the last picture), it is ready (Mine ran for about 10 minutes). Remove the tyvek from the still full machine. You'll need to remove the bag foot first so it will drain. Be prepared to get wet, as the tyvek will not get wet, and all the water on it will drip off. This water should be perfectly clean, so feel free to re-use it for washing your clothes...

Take it outside, shake it out and let it dry. (be prepared to get wet!) Once the outside is dry, you'll need to invert it again, to dry the inside. Once dry, invert it again, and take it to your work area.

Step 9: Velcro

At this point, your bivy is almost complete. Add a few velcro spots, and it can be opened or closed easily. If you're looking for better warmth and weather resistance, you can add a full velcro strip, or use Tyvek Tape to attach a full zipper.

Step 10: Complete!

You have successfully completed the basic bivy.

Feel free to trim the top edge, (rounded looks nice). but either way, it's ready to use. It folds up nicely, and fits my adult mummy bag well.

Author's Notes:
The carpet tape and tyvek brand tapes are amazing stuff. (A whitewater raft manufacturer recommends the Tyvek tape for field repairs.) The glues on both of these are strong enough to separate the lat\yers of Tyvek, and some thrifty folks use tyvek and this tape to make Boat sails. The seams are difficult to pull apart with the carpet tape, but if you pull the exposed edges apart, the tyvek will fail before the tape will.

The bivy is surprisingly warm by itself, and in a rainstorm a clothed person could crawl in for an emergency shelter. Maybe a comfortable bag liner would be all you'd need in summertime...

Ideas for improvements:
You can seal the full lenght of the bag, and cut the opening down the middle. With a smaller strip of Tyvek, you can add a flap that would overlap (seam with tape on one side and the bottom. Using velcro as a closure, this might work to be more weather resistant.

Most bivys have a drawstring, so you can pull the bviy around your head. Sewing tyvek is hit and miss, as the punctures act like perforations, and are easily torn out. Perhaps a pocket for the head would work better...

For Mummy bags, you can taper the foot area to use less material, and save some weight. As constructed: 10.8 oz, 108g. Total cost. about $25. (with lots of tape left over).

I left the top flap longer than I need. In the event of rain, I can pull it over my head, and avoid getting too wet. With some ingenuity, and some mosquito netting, a complete foul-weather bivy could be constructed.

Tyvek is relatively cheap, and the possibilities are endless! I'd love to see design improvements!

Thanks to my 10 year old model: Sarah for her Bivy demonstration...