I was wondering if I could make a bivy with a Gossamer Gear polychro groundsheet bottom sewn or bonded to a Momentum top and hood. The polychro would be lighter and less expensive than a silnylon floor, easily repaired with tape, and (when needed) replaceable. Anyone tried this? Am I crazy? I have been very impressed with how durable the seemingly fragile polychro really is (when used a groundsheet). Because it is a shrink-wrap type plastic, it seems to stretch rather than puncture.

> Am I crazy?

Maybe. But you have company.

I've been considering doing the same thing but with some Spinnsheet material I've never used, instead of Momentum. (Seeing what I can do with some unused materials.)

I'm still mocking up the bivy design with some old sheets so I haven't gotten to any Polycryo construction testing. But the next phase will be testing various stitch patterns to see if the Polycryo will still have enough strength at the seam. If not, I'll be looking for at least a new floor material.

Polycryo is hardly an ideal material, see:

http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/6953/index.html?skip_to_post=49630#49630

But I have it on hand so ...

My biggest concern is that this will only result in a disposable bivy.

On the other hand if it works, it will be very light.

Good luck.

I have 2 of these ground cloths. When I first got mine I weighed it to see what the square yard weight was. It is 0.568 ounces per sq yard.

I thought about making other things with it but I have 0.33 ounce per sq yard Cuben. Lighter but does cost more.

The stuff is strong and could make a Bivy and should last as long as one made out of light Cuben. I don't know how well it might sew or if any of the glue stuff would work on it. I might try sewing a Bivy out of one and see how that works.

I have been playing with a very interesting material for packs. It also is a bit heavier than Cuben but is very strong and more or less free. It weighs 3.4 ounces a sq yard `so a decent size pack might weigh about 1 pound.

I have thought about a gear list out of really cheap or recycled materials that while it might weigh a bit more than Cuben the cost would be about nothing.

The gear list would still fall into the SUL weigh requirement and be under 5 pounds.

I've done this before with a nylon top and it worked great.

The polycro is less wind-proof than the nylon breathable top (as weird as that sounds), so make sure you have enough Momentum so the polycro doesn't rise up much when you are in the bivy.

But I think the idea is generally sound and should work.

Consider some cuben tape (from zpacks.com) if you don't want to sew it.

I guess I feel encouraged to proceed. So, I've ordered the materials from GG and Thru-hiker. I'll sew the Momentum top, which will include a Nanoseeum bug net (similar to a Ti Goat Ptarmigan) and a side zip, then when that's finished, I'll either sew (if it seems possible) or bond a piece of polychro 24" by 78" to be the bottom. I'll just do one or two lines of stitching so that it will be easy to replace. If it doesn't work out, I'll switch to silnylon or a GG spinaker groundsheet or cuben for the bottom. The polychro is definitely the cheapest UL option, though cuben would be nice. I'm hoping the finished product will be 5 oz or less, which would mean I'm only adding 3.5 oz to my kit (since I use a medium polychro groundsheet at 1.5 oz already.

If anyone has a piece or pieces of cuben that would make a 24" by 78" bottom out of 1 to 3 pieces, let me know what you would want for it.

Follow-up comment:

Tried a single line of stitches to attach a test piece of polychro to nylon ripstop. I used a fine needle, widest spacing on my primitive machine (probably ~12 per inch) and the result seemed strong and would only fail if I really pulled pretty hard. I think with some care a bivy would be pretty durable (at least in terms of the risk of the polychro to Momentum attachment breaking down).

I'll post some pics when I finish the project. Materials arrive today and tomorrow.