Tyvek Bivy Sack for a Big Guy
Updated 01/18/2011




Introduction

A bivy sack is more than a sleeping bag cover, less than a tent. It must seal out the elements, yet let in fresh air. Its material should be both breathable and water-repellent. It also must be extremely light (otherwise you'd just carry a tent). In addition to being lightweight, a bivy offers the advantage of not needing to be staked down.

Most bivys are basically tubes, closed off at one end and somehow split at the other end to provide ventilation and access to the sleeping bag. This can entail a zipper or a drawstring, and sometimes a piece of screen. The bivy's water resistance, weight, and durability are determined by the choice of material and construction method. It's all about compromise. Performance, weight, and cost—pick any two!

The main problem is keeping the top of the bivy off your face. Some designs have no rigid structure at all, but most use at least one hoop. Tents typically use hoops of sectional bungee pole, but I haven't seen that on a bivy. (Maybe bungee pole doesn't scale down well.) One commercial design I examined uses a length of semi-flexible, lightweight Delrin plastic "wire" for the hoop. It seemed flimsy and was irksome to work with. I also worry about repeatedly flexing plastic in extreme cold.

None of the bivys I've seen, including some very expensive ones, are big enough for me.

Several campers have constructed bivys out of Tyvek, using a variety of techniques. This blog was my introduction to the concept (more below). Tyvek offers light weight, decent breathability and water resistance, and low cost. The downside is that Tyvek, or the soft variety at least, is not very durable.


Design and Planning

Material Concepts sells perforated Tyvek, like that used in Frogg Toggs-brand clothing, in rolls of 36 inches by 10 yards for a reasonable cost. They also provide free samples of six types. I tested each for gas- and water permeability and found one that seemed to offer the best compromise: type 1673, at 2.1 ounces per square yard. It's thin and soft, cuts easily but resists tearing, and sews well.

My bivy is a box shape, with the top sheet ending about nose level and the floor continuing back and then looping up over the top. This overhang drapes over a hoop that is set into the ground perpendicular to the bivy at the sleeper's eye level.

  
   Figure 1: Drawstring connects grommeted corner to stake; length is adjusted with cordlock.
The outer corners of the overhang are staked down from inside, with lengths of string, adjustable with cordlocks, connecting the stakes to the corners (Figure 1). You could instead use Velcro to attach the corners of the overhang to the top sheet, but I find that Velcro doesn't perform reliably in the snow.

After drawing preliminary plans, I used a printout to create a paper model. It seemed to work, so I proceeded with the construction.

The finished envelope weighs 1.5 lbs.; the hoop kit weighs 1.25 lbs. The plans (Figure 2; click on image for PDF file) provide most of the information someone would need to make this bivy, but construction details follow below.



Figure 2


Envelope

The roll from Material Concepts is nominally 10 yards long, but my roll contained enough extra to make two, 36" x 10" extension "wings" for the overhang plus a drawstring bag to hold the hoop sections and stakes. Lightweight cotton-polyester thread was used throughout, plus a little bit of duct tape here and there to reinforce corners. No seam sealer was used, as this bivy is meant for below-freezing conditions, where breathability is more important that imperviousness.

With the plan being to turn the envelope inside-out after sewing, the zipper was located on the "outside" of the envelope and on the side opposite my sleeping bag's zipper, so that when the envelope was turned out, the zipper ended up in the right spot.

The exposed, unfinished edges—the end of the top sheet, the end of the overhang, and the overhang's wings—began to fray soon after assembly. It would have been better to have hemmed the top sheet prior to sewing the pieces together, but after the fact I edged it with duct tape. The "wings" were hemmed 1" at the corners, tapering back to about 1/4" where they meet the sides. The front edge of the overhang was hemmed 1", then rolled back and hemmed again, making a lip three plies thick, six plies at the corners. This was done to support grommets in the corners, and also hopefully to make that edge less likely to flutter in the wind.

After construction, the upper half of the envelope was lightly spray-painted to be visible in the snow.


  
   Figure 3: Hoop assembly with base-tube extensions in place. Insets show folded sections (top) and Tyvek drawstring bag.
Hoop

The hoop (Figure 3) is made from about 6 feet of 1/2" OD aluminum tubing and about 3 feet of 5/8" OD tubing. The smaller tubing is cut into four arcs; the larger tubing is used to make two anchor tubes and three short sleeves where the sections join.
Because the small tubing fits loosely inside the larger, I wrapped the ends of the small tubes with 7" to 8" of aluminum tape to take up slack.

The hoop sections are permanently fixed to connecting sleeves with 1/8" aluminum rivets. A bungee inside the tubes holds the assembly together.

After joining the hoop sections and base tubes, I bent the assembly, comparing it to an arc I had drawn on butcher paper. The bungee went in last.

The base tubes are intended to be driven about 6" into the snow; extensions made from the small tubing can be added if needed, and are attached to the bottom tubes with quick-release pins (McMaster-Carr p/n 98404A007).


Tools

The envelope required a sewing machine, tape measure, yardstick, pen, scissors, and a bit of duct tape and spray paint. Making the hoop entailed a bench vise, drill press (a hand drill will do if you're careful), tubing cutter, rivet gun, small half-round file, string to thread the bungee cord through the tubes, and a surface to bend the hoop assembly against—I used a spare tire of 26" diameter for the main bends and a round part of the bench vise for tweaks closer to the connector sleeves.


Field test

The system worked as planned. I don't know whether the bivy contributed any R-factor, but it did shield the sleeping bag from wind and falling/blowing snow. The spaces around the hoop bases allowed sufficient ventilation. Some frost accumulated inside the domed space, especially the night the temperature fell to -30°F, but I think that's unavoidable. I ended up using only one hoop extension to make up for uneven snow depth. One drawstring failed; next time I might use thin bungees.


More Homemade Gear

[sled system]  [minor gadgets]


Other Tyvek bivys

http://www.instructables.com/id/Tyvek-Bivy-Sack-for-CampingHiking/
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=12370
http://ezinearticles.com/?Experiences-With-a-Tyvek-Bivy&id=3143390
http://sirenbicycles.wordpress.com/2008/07/06/tyvek-bivy/
http://traversejapan.wordpress.com/2010/07/14/whats-cheaper-lighter-and-more-breathable-than-an-e-bivy-my-myog-tyvek-bivy/