First of all, I want to thank all those that have shared their insights on this thread, all those posts have been extremely helpful in my first attempt at sawing a bivy. I though I would add my experience since I have deviated a bit from previous posts (I think the beauty of making your own gear is to be able to customize its dimensions, etc).
I basically followed the guidelines posted in this thread (I.e boxed corners, horizontal netting at face level, etc). What I did a bit different was:
1) Closing: I put a zipper running lengthwise along the middle of the bivy. I think this makes entering / exiting the bivy much easier. The added weight would be marginal if you consider the total length of having a zipper across the chest plus a side zipper.
2) I kept the girt of the bivy wider from shoulder to hips. (i.e 8 sides instead of 6 sides). This is similar to what Joe at Zpacks is doing with his splash bivy.
3) The walls have a larger section of waterproof material than beathdable (8” silnylon + 2” argon). This made the construction a bit more difficult but not too bad.
4) I put tie outs in the corners and on reinforcement patches where the zipper ends. Not sure I will actually use the corner tie outs but I though it would be much easier to put them on at the time of building the corner seams that trying to add them on later. Their weight is negligible.
Again I'm not very experienced at sewing so I found it very useful to baste all my seams before the actual sewing. This was time consuming but made it much easier to sew the actual seams on a slippery material like silnylon. To figure out the dimensions that would work for me, I did first a full size mock up with an old sheet I bought at a Goodwill store. This was very helpful to make some adjustment of the size of the final product (this is when I realize having a larger girth at the hips level was beneficial). Sawing a mock up on an easier material like a cotton sheet also allow me to get a little practive before I transitioned into a harder material to work with like silnylon.
Here are some basic info on my bivy:
Materials used:
Top: 0.67oz/yd Argon (From Dutchwear Gear)
Bottom: 1.35?oz/yd Silnylon (Actual weight more like 1.7 oz/sqyd) (Seconds from Oware)
Netting a Face level: 0.7oz/yd ultralight insect netting (From Zpacks)
Zipper: 56” #3 Zipper with double tab
Thread: Gutterman Tera 70
Needle: Schmetz Microtex 70/10
Measurements:
Top Girth (Head): 64”
Shoulders Girth: 78”
Hips Girth: 78”
Bottom Girth (Feet): 64”
Length: 90” (Laying out flat)
Walls: 10” (8” of waterproof, 2” breathable material)
Netting at face level: 9" strip
Total weight came in at 7.8oz. This is much heavier than what I was hoping. The silnylon I used was definitely much heavier than it's nominally spec'd weight (1.3 oz/yd). I weighted that part alone before assembling and it came in at 5.8 oz, so based on the dimensions I was using the actual weight of the silnylon was more like 1.7 oz/yd). I hope this means I have a more durable bottom. Overall I'm fairly happy with the final product. I will be testing this bivy next week on the field and will be able to report on the breathability of the argon 67.
I will be using this bivy with a Medium Exped synmat 7 mattress (which measures 22” x 72” x 2.8”, actually Exped shows 20.5” width in their website but mine measures 22” inflated… not sure if they have changed their width specs since I got mine).
On this pictures I have a Western Mountaineering Versalite stuffed with some small pillows to simulate a person inside. I hope this helps.
Edit: Pictures added
