100% polyester bivy top?

Sorry, I failed to mention two things

1) The 100% polyester isn't like what you see as a 100% polyester shirt or whatever. This polyester appears to have a ripstop characteristic to it (would there be such thing as ripstop 100% polyester? I don't know), and I do not question the durability of it to tears or abrasions. Just looking at it and feeling it, the fact that it is porous is surprising to me — but I suppose the pores are just big enough as it will pool water yet slowly make its way through.

2) I do not see this project as a permanent solution. I seek to practice the skill of making my own gear on el cheapo materials, determine my resistance/acceptance of bivy tarp camping without a large investment; and, frankly, I just need a productive indoor hobby for when I get home from the office. I would likely only use the bivy on 1-2 nighters, but I'm curious if you all would expect it to last multiple seasons before going sour.

I've seen polyester rip-stops with a mini-grid pattern before, usually a 1.9 oz fabric weight, or even heavier. It won't be the lightest bivy unless you found some rare polyester ripstop around the 1 ounce weight. And just because Walmart says it's polyester, doesn't mean it couldn't be nylon either.

As far as using a breathable, non-DWR coated fabric in a bivy, a lot depends on atmospheric conditons as to how it will perform. On a foggy night moisture will condense on anything. On a cold night the dew pint could be inside your bag or bivy and collect condensation.

Polyester theoretically does dry quickly and has less water absorption than nylon. If after building it you find it needs some water repellancy, you can wash in Nikwax Polarproof or one of the fluorpolymer dwr's such as Grangers, Atsko Permanent Waterguard, or 303 (stinky solvent carrier, but works well).

Putting something waterproof like silnylon near the opening you breath out of is also a good idea if you'll use this bivy below freezing.

>"Am I overlooking other cheap, outdoor-worthy (durable) fabrics that are more wind-resistant and lighter than polyester but with enhanced breathability?"

Momentum 90 at through hiker is light, reasonably durable, good initial dwr, but not cheap.

Strong, cheap = canvas (or cast iron), but heavy. Strong, expensive = spectra (titanium), but not cheap. Light, cheap = silk (balsa wood), but not durable.

Throw in cosmetic factors for materials (a nice hand, good drape, colors, texture, etc.) and your choices can get even trickier. Big gear companies usually opt for looks and durability that sells, and price and weight (and often functionality) takes a back seat.