Bivy sack material

I would go with something like the Momentum available at thru-hiker.

for under a tarp ( i am assuming full coverage tarp) you don't want waterproof. breathable is the answer. Momentum as Chris answered is the A#1 choice. However if the $14yd price is too steep for you check OWFINC.com they have breathable 1.1 rip for a lot less. I like the red, it's cheaper becuase they think it's a 2nd. I can't figure out why in the 20 some yds i've ordered of it. In fact Chris has a bag i made using this exact fabric. I order bulk and pay under $4yd for this. Always find a way to order 20yd of 1 type of fabric from them to get wholesale on your entire order. A great savings, you just have to plan all your projects at once, and fund them too.

-Tim

My wife just completed sewing the SMD Meteor bivy for me last month. I used momentum (from thru-hiker)for the top and coated mini-ripstop (purchased on sale at rainshed) for the bottom. Silnylon would've been slightly lighter for the bottom but not as waterproof as the urethane coated fabric according to my conversation with the owner of thru-hiker. Apparently while silnylon is fine for a tarp, when it is laid on with pressure there can be some water pass through the fabric. My bivy came out weighing 9.5 oz nonetheless. Be advised sewing with momentum and silnylon can be a tricky affair with these slippery fabrics. Experiment with stitch length and tension on scraps first.

I've had good luck with floor/gs's made from the double-coated silnylon from americanhomeandhabitat.com. It has a coating of silicone on each side. The first pass impregnates the nylon weave, the second forms a uniform film. Regular silnylon will seep if the ground is wet. Not a problem if you use your bivy with a tarp, but water will usually run underneath an unprotected bivy unless you have the gumption and opportunity to put it on thick forest duff.

Be warned that polyurethane also will seep unless the coating is thick (and heavy). That's the logic behind Super K-coat from Kenyon being used for tent floors.

I rely on my closed cell foam pad inside my bivy for extra
water proofness. The bivy keeps me on my pad this way.

Most silicone coated nylon is coated both sides. It is also
hot calandered to a degree. The amount of coating and the
solvents used to apply it are major differences as to
water proofness under pressure.

I made two Meteor Bivys using Silnylon for the floor and Momentum for the top.

They work great … I too put my Foam Pad inside the Bivy so a little moisture is not a problem.

It's a great little design ….. I'll be using it on the AT this fall.