Wednesday, August 20, 2014

DIY backpacking gear - Tyvek bivy bag

Tyvek bivy bag in "sleep under the stars" mode
Bivy bag in "inclement weather" mode
Every time I go backpacking, I keep thinking - "What more can I do to lighten my load?".  Because I live in Southern California and many (most?) trails don't have reliable year-round water supplies, we're forced to take large (heavy!) supplies with us, so the more we can lighten the rest of our loads, the better.

This time out, I was after lightening my 4 lb. "shelter" (tent and footprint), using a DuPont™ Tyvek® sheet made into a bivy bag.

Advantages:
  • Lighter - bivy bag and groundcloth is 1.5 lb., vs. 4 lb. for my 1-man backpacking tent & footprint.
  • Warmer - because it's such a tight weave and is so close to you, it traps body heat much better than a tent.  Claims of an extra 5°-10° F are probably not unrealistic.
  • More compact - folds up smaller than my 1-man tent and footprint.

Parts list:

My Tyvek was shipped in Tyvek!
(good for small scraps)

  • 1 9'x12' Tyvek sheet (~ $25.99 on eBay), available from various sellers on eBay - this is the one I used and can highly recommend.  If you don't need a Tyvek groundcloth, you can get a 9'x8' sheet.

    Also, I used the standard "house wrap", mostly because that's what I saw more recommended by others.  But some use the Tyvek 1443R "soft structure", which is lighter softer and quieter, but more expensive and more difficult to find in the necessary size.  I was a bit concerned about its water resistance, since I plan to use this bivy by itself, without a tarp.  If you make one from this fabric, please let me know in a comment how it works for you...
  • 1 roll Tyvek tape ($12.50 or so from eBay)
  • 2 36" robe zippers ($2.99 each at Joann Fabric and Crafts).  You can use most any 36" zipper; this was the only one I could find locally.
  • 1 5' length of 1/4" PEX pipe ($1.76 at Lowes)
  • 1 2" piece of double-sided Velcro ($4.99 for 25 8" strips at Joann Fabric and Crafts)
My pattern
(click to enlarge)

Assembly Instructions:

  1. Make yourself a pattern.  I used this (dimensions are in inches), but you'll probably want to change the dimensions to fit your bag, height and girth.  I'm 6'5" and have a very long bag, so I made mine using the full 9' length of the Tyvek.  If you're shorter, you can trim to make it smaller and save some weight.
  2. Knead and/or wash the Tyvek to soften and quiet it.  There's differing opinions on the 'net as to whether it's better to soften the Tyvek before assembly or after.  I chose to soften first (both by kneading the dry material by hand for 10 minutes or so and then putting in a washing machine (with no detergent or fabric softener, water only!) for a full cycle.  This has no impact on its water resistance.  Note that doing this will "shrink" the Tyvek by a few inches in both directions, so if you plan to wash after assembly, be sure your pattern is upsized to accommodate this.  Mine shrank from 12' to 11' 4" and from 9' to 8'5" or so.
  3. Using your pattern, mark your cut points on the Tyvek, using either a pencil or fine-point permanent marker.  A yardstick can help keep the lines straight.
  4. Make your cuts.
  5. Attach the vertical zipper.  Set your sewing machine to use the longest distance between stitches.  You'll need to cut a 3/8" slit on both sides at the end of both zipper cuts in order to fold back 3/8" of Tyvek.  This makes an opening for the zipper and allows you to sew through two layers of Tyvek, making a stronger attachment.  Take your time, this is probably the hardest step.
  6. Zipper detail
    (click to enlarge)
  7. Attach the horizontal zipper.  If you got a single-sided zipper, like me, you'll want to attach this one with the pull tab on the inside of the bag, to make it easy to open from the inside.
  8. Using the Tyvek tape, seal all of seams of the bag.  You can do it in most any order, I opted to tape the long vertical seam (above and below the vertical zipper) first, followed by the top and bottom of the bag.  I opted to put about 1/4" of Tyvek overlap on all of my seams (with the overlap coming from the top of the bag, to keep any rain out that might get under the tape).
  9. If you want, turn the bag inside out and put a layer of Tyvek tape on the inside of all of the seams.  This will help strengthen the bag and keep the overlapped flaps from catching on anything inside the bag.
  10. Get inside of the bivy with your gear and the PEX pipe.  Figure out where you want to put the pockets for the ends of the PEX pipe and where you want the Velcro strap.  Mark the locations with your pencil or marker.
  11. Make pole pocket by taking a 1/2" x 1-1/2" piece of Tyvek and placing it vertically in the middle of a 3" piece of Tyvek tape. Place it centered over the mark you made in the previous step.  Repeat for the other side.
  12. Cut off a piece of Tyvek tape 1/2" x 2".  Use it to attach the velcro strap to the marked position on the inside of the hood.
  13. Cut the PEX pole as needed to fit inside of your bag.  I needed to trim about 6" off of mine - you may want more or less.
Velcro strap
holding PEX pole
(click to enlarge)

PEX pipe inserted
into pole pocket
(click to enlarge)

2 comments:

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