DIY Lightweight Bivy for Wind, Bugs, and Splash Protection--Seeking Advice

[MitchellK]

I've been wanting to start sewing my own gear for a few years now, and I think that a UL bivy would be a fun and useful first project. I've been wanting something light, breathable, and NON-waterproof that I can use underneath a poncho-tarp to protect from wind, bugs, and any rain splashes that get in.

My current thoughts are a 3-part design similar to this: https://diygearsupply.com/diy-guides/bivy-sack/ but I plan to take my own measurements, etc. The materials I'm currently planning on using are
- 20D 1.1oz/yd silnylon for the floor (maybe 30D for added durability)
- 0.34oz/yd Dyneema Composite Fabric (DCF) for the foot-to-stomach "ceiling"
- no-see-um netting for the chest-to-face "ceiling"

Some things I'm worried about though that I'm hoping people will have experience / opinions on:
- Would you expect condensation from using DCF for the foot-to-stomach portion instead of something more breathable? I'm hoping that the giant netting-only portion will help combat this issue, but will it be enough?
- Are there any issues that I need to look out for when sewing DCF to silnylon? I'm not too worried about waterproofness of the seam for this application, but I do care about durability.

I'm excited to try making my first piece of large gear. Hopefully all goes well, I'll definitely be practicing with cheaper materials before I commit to the real thing. I'm also curious to hear about anyone else's MYOG experiences, especially regarding sleep systems!

[Wandering Daisy]

Sorry, but what is DCF?

As for condensation, in any bivy you get less if you oversize the width. Maybe put in a light weight zipper and only enclose the lower section when it is raining. Also depends on your sleeping bag. If it has water resistant down or shell, then I would not worry much about condensation.

The issue with the bivy-tarp combination is that there are many UL tents that actually would weigh less and off just as much protection.

But if you just want an easy project to start for making your own gear, the bivy is a good idea. You may consider experimenting with an old sheet (sew with very wide stitch that is easy to undo) to test the size and then use it as a pattern when you cut into the more expensive material. Unless you intend to just buy a pattern.

[MitchellK]

Thanks for the tips! I was already planning no using an old sheet, but I hand't thought about using the wide stitch so that I can undo and try again. That could potentially save me a lot of sheets when things inevitably go wrong!

DCF is Dyneema Composite Fabric, it used to be called Cuben Fiber. It's what many of the high-end tents are made of (although notably, those tents are often made from a slightly thicker version of DCF that I'm intending to use). I updated the original post so that it spells out the whole fabric name .

My hope with the combo is to slightly reduce the total (sleep system + walking rain system) weight as well as to experiment and see whether I'm more comfortable in a UL tent or a poncho-tarp + bivy system. But yeah, there are some extremely light tents out there that could negate pretty much any weight savings I might achieve.

[bobby49]

I used to fool around with DIY bivy sacks like that about forty years ago. However, fabric materials have gotten much lighter now, so there is very little to be gained with a tight bivy sack. Now, for backpacking purposes, I use a DCF/Cuben Fiber shelter that weighs about 8 ounces for the main structure. Additionally, I use a trekking pole as a center pole, and I have a ground sheet that weighs 2 ounces. Add in some titanium stakes. The practicality of this is that I can get totally inside with all of my gear and wait out a rain storm. Been there. Done that.