Subscribe Contribute Advertise Facebook Twitter Instagram Forums Newsletter
MYOG sub 4 oz side zip cuben/momentum 55 bivy.
Display Avatars Sort By:
Andy Duncan
(bluewater) - M

Locale: SoCal
MYOG sub 4 oz side zip cuben/momentum 55 bivy. on 02/27/2012 17:27:24 MST Print View

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 ( and the Momentum 55 was available at Thru-hiker ( 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 (

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.

Ryan W
(mwilks) - F
Nice! on 02/27/2012 17:52:25 MST Print View

Right on! That looks great. Impressive work and way to get out and use it.

Jace Mullen
(climberslacker) - F

Locale: Your guess is as good as mine.
MYOG sub 4 oz side zip cuben/momentum 55 bivy. on 02/27/2012 18:08:29 MST Print View

Thanks for posting this up Andy!

I will be emulating your design, hopefully I'll get some time over spring break to assemble it. I might go with .74 Cuben though. Just for a little bit more longevity.

Again, many thinks. That pictures and dimensions will help a lot when I'm trying to figure out mine.

Andy Duncan
(bluewater) - M

Locale: SoCal
MYOG sub 4 oz side zip cuben/momentum 55 bivy. on 02/27/2012 23:22:43 MST Print View

Thanks for the comments. Good luck with your bivy project Jace, and have fun on the PCT this summer. Andy.

Ultra Magnus
(Ultra_Magnus) - F
Re: MYOG sub 4 oz side zip cuben/momentum 55 bivy. on 02/28/2012 15:15:57 MST Print View

That is one beautiful bivy. I made one loosely based on the SMD Meteor patter a while back, and I found transitioning from the french seam to the zipper very challenging, at least for my sewing skills. Could you post a close photo up of that junction?


Phillip Colelli

Locale: AT, follow@
Re: Re: MYOG sub 4 oz side zip cuben/momentum 55 bivy. on 02/28/2012 15:29:57 MST Print View

I really like how you made the cuben come up the sides a full 8". I have she MLD superlight which I'm sure you know is very similar and the cuben only goes a few inches up, partially due to my 20" wide neo air taking up all the interior space.

When I first opened the thread I was like darn that sure looks like a superlight... but then I looked harder. 3.9oz is very impressive! I suppose the weight savings coes from using the .51 cuben instead of .75. Pretty smart trying to use as much cuben as possible to reduce weight further.

You may just be in possession of the lightest bivy in the world! (maybe)

Jason G
(JasonG) - F

Locale: iceberg lake
nice on 02/28/2012 19:37:48 MST Print View

looks nice. What was the total cost of materials?

Andy Duncan
(bluewater) - M

Locale: SoCal
Re: Re: MYOG sub 4 oz side zip cuben/momentum 55 bivy on 02/29/2012 14:19:53 MST Print View

Thanks for your comments Ultra. For the junction where the zipper ends and the side/french seam begins I left the fabric that is on either side of the zipper, but cut about 1/4" of the actual zipper off. Next I used the remainder of the zipper fabric along with the surrounding fabric to make the french seam in that area. I found that by removing 1/4" of the actual zipper and keeping the zipper material on either side it was easier to create the french seam. The zipper itself does not seem very flexible and was difficult to incorporate it into the french seam. I'm not sure if that makes sense, but please let me know. A few pxts are below.

Many thanx Phillip, it's nice to know other people are totally into the details. Only MYOG'ers on BPL would notice the use of as much of the 0.51 cuben on the sides to save weight. Right on! This may be the lightest functional bivy, although there was a MYOG post a while ago for a 2 oz bivy using 0.34 cuben and Momentum 50 w/ no zippers etc. Man, it never ends.

For Jason, the cost of the materials is an estimate, but here's my best guess:

3 yards 0.51 cuben @ $26/yard: $78
2 yards Momentum 55 or similar: $24
7 feet continuos coil zipper tape @ $0.40/foot: $2.80
2 double pull zipper sliders @ $0.35/each: $.70
2 feet 1.25 mm Z-line @ $12.95/50 ft: $12.95 (I had some extra laying around and it's only available in 50 ft length, probably easier to just use some string)
18" x 36" of .7 oz/sqyd ultralight netting: $8

Total w/out the Z-line: $113.50



James Berwick
(jhb0510) - F
Length on 03/24/2012 02:37:54 MDT Print View

Hi Andy
How long is your Bivi?

Andy Duncan
(bluewater) - M

Locale: SoCal
Bivy Dimensions on 03/25/2012 12:22:14 MDT Print View

Hi James, the bivy is 78" long. This seems to be enough to fit a winter quilt, some extra gear and a pillow at the head end (I'm 6' tall).

James Berwick
(jhb0510) - F
Sil Nylon Bivi on 03/29/2012 02:53:08 MDT Print View

Thanks for that Andy, I am just about to start on a Silnylon one, as it is my first proper MYOG project, I'm not going to spash out on Cuben! I am a little bigger than you, so I might strech your measurements a bit.

michael levi
(M.L) - F

Locale: W-Never Eat Soggy (W)affles
awesome.. on 09/20/2012 19:15:16 MDT Print View

1 question, how is the breathability with the M55, compared to the M50 or M90?

Andy Duncan
(bluewater) - M

Locale: SoCal
MYOG sub 4 oz side zip cuben/momentum 55 bivy. on 09/24/2012 21:21:43 MDT Print View

Michael, I've been in the Sierras and just saw your post. The breathability of M55 feels very similar to M90. I recently used the newer M50 for a windshirt and summer quilt and although it has a good DWR it does not seem to breath as well.

I really like the Pertex Quantum available at Zpacks right now. It's slightly heavier but breaths well and feels more durable.

The bivy was a life saver during a desert windstorm last winter. This summer I've transitioned to using a slightly larger quilt and tarp with no bivy. This seems to save a few ounces while increasing the temperature comfort range.

I used a MYOG 20 degree quilt with a Pertex Quantum shell and 7D liner without a bivy and a 6' x 9' cuben tarp on the SHR this summer and felt fine. I 'spent' the weight of the bivy on a slightly heavier but more windproof shell material, larger quilt dimensions and 1.5 oz more down. After a 23 degree night in Upper Basin last week there was a heavy layer of frost on the outside of this newer quilt (while not using a tarp). No problems with the DWR and the extra down kept me comfortable.

Good luck with your bivy!