After reading many of the MYOG posts on BPL I started to get comfortable making most of my gear last summer. This is my humble attempt to give back a little to the online BPL community.
My goal was to make a bivy with a half length side zipper that weighed under 4 ozs while still having plenty of room for my shoulders/elbows and an inflatable sleeping pad inside the bivy. I also wanted something waterproof on the bottom with material that was durable enough to use without a groundsheet. My polycryo groundsheet was about 1.5 ozs anyway so if I could do all of this and keep it under 4 ozs I figured it was worth the effort.
I used 0.51 oz/sqyd cuben for the bottom and 0.75 oz/sqyd Momentum 55 for the top. The cuben is available from Zpacks (http://zpacks.com/materials.shtml) and the Momentum 55 was available at Thru-hiker (http://thru-hiker.com/materials/breathable.php). Thru-hiker only carries Momentum 50 or 90 now so ymmv with the newer material. I would probably use their lighter stuff today (Momentum 50). The zippers and other hardware are available at Quest Outfitters (http://questoutfitters.com/).
The bottom has an 8 inch bathtub floor all around. The idea with the tall sides is to use as much of the lighter weight cuben as possible and to provide some side protection from rain spray. The top has a slight 2 inch inverted bath tub (for lack of a better term) and there is an increase in height in the middle down by the foot end. The 10 inch total height seems to work for me.
All of the seams are sewed using a french seam except the net/zipper area which has a double stitched flat felled seam.
I used this bivy on a JMT thru hike last summer. Unfortunately there were storms during first four days. I slept under a 5.5’ x 8.5’ tarp and it worked out great. I sewed a small tab to the head end so I could attach a cord and lift it off my face, but I found it really wasn’t necessary. The net area seemed to be located just right and the material was kept off my face by my hat.
I used an xl Kookabay inflatable pillow, GG torso pad along with a Kookabay Taperlite inflatable pad on top and a GG sitpad for my feet. This all fit inside with plenty of room for me and a puffy jacket. There is not enough room to fit a larger 2 ½ inch deep inflatable pad like a Neo-Air inside, although if needed I would just put it on the outside. I have tried the small, medium and large Neo-Air’s and finally found the ccf torso pad/Taperlite combo works best for me.
The 0.51 oz/sqyd cuben seems to work fine as a ground sheet. I haven’t had any problems with leakage or unusual wear and tear.
The Momentum 55 is breathable and did shed some rain spray. Condensation was only a problem when I slept on the shore of Cathedral Lake with the cold wind blowing toward me off the lake. There was ice on the outside by my feet in the morning, but I was warm (and dry) inside.
When it wasn’t raining I just slept under the stars. On the last night I slept on the west side of Whitney (by the tarn above Guitar Lake) and there were some heavy winds. I could feel my legs and feet being moved by something, but I couldn’t feel any change in temperature inside.
I was a little unsure about using a bivy before the trip, but now I’m completely sold. I used an 18 oz quilt and slept warm and toasty (I wore an upper base layer, a windshirt and sometimes a puffy jacket, but only shorts/socks and no lower base layer to sleep).
The dimensions are about 22” at the head tapering to 32” at the shoulders then tapering down to 24” at the foot end (all with the 10 inch height). To give some perspective I am 6 feet/190 lbs. Having enough shoulder/elbow room was important and it seems to have worked fine.
Camp by Palisade Creek overlooking Deer Meadow and Devils Crags.
The obligatory UL scale photo.