MLD Superlight bivy with cuben floor – durable without ground sheet?

I would go silnylon out of these choices, but it depends on your intended use. Camping on rocks or gravel will abrade the 0.75oz cuben quickly. If you're camping on softer surfaces it'll last longer. I personally view cuben with the heavier .18 mylar layers (ie. 1.0oz cuben or greater) as the minimum for use as floors, but even that is questionable IMO. Silnylon will definitely last longer. The choice depends on your willingness to care for it and your tolerance for replacing gear.

I am looking at a 3oz difference between silnylon and cuben on the large. I need the large because of my NeoAir but I am a gram weenie.

I wonder why MLD use this stuff if its so weak. I assume its because you can make it work if you try hard?

"I am looking at a 3oz difference…"

You are not looking at a 3oz difference, because if you go the CF way, you would need a groundcloth layer, which will add to the weight and diminish the difference.

"I wonder why MLD use this stuff if its so weak"

MLD makes this stuff because there is a (limited) market for it. People who would not use it on abrasive ground surface are one set of such customers. If your intended use is in such conditions, you fit in this customer profile. Otherwise, get the silnylon as a proven time-tested durable solution. Save the grams elsewhere.

There are lots of cuben floor haters out there. I have used cuben floors in both the .74 oz and 1 oz flavors for the last 4 years. The final result, just this season, is I noticed one tiny pin hole. The .51 cuben repair tape is cheap, readily available and so simple even I can use it for cuben repairs. I think just using good site selection and preparation and understanding and accepting the limitations of the material and you will be fine. This is true of any cuben product. You won't wake up floating in a cuben bathtub of rain water. I doubt you will burst into flames. If it starts raining frogs, stay inside your cuben bivy until it passes.

P.S. If anyone wants to sell their very dangerous, ridiculously fragile cuben bug bivy, please PM me!

Great reply!
I am now ordering a cuben floored bivy.
Thanks for setting me straight based on your experiences.

It is really is what Dan said, if you can accept a little more sight selection, prep and care, and realize you won't be passing a cuben bivy to your grandkids along with a Rolex and a McHale pack, save the ounces. I met a PCT through hiker last year in the Glacier Peak Wilderness who was using a cuben bug bivy and he raved about it. I hope to get one soon to use with my Gatewood Cape. Get some .51 cuben repair tape from Zpacks and carry a bit of that in your kit.

I don't use a groundsheet with my Superlight. I don't have a lot of nights in it, maybe a dozen, but it's holding up well. You mention using a NeoAir, but for those who use a closed cell foam pad it's a no-brainer. Just put the pad on the outside of the bivy.

I have however recently transitioned to using a torso-length Prolite on the inside and a ridiculously thin (GG Thinlte 1/8") evazote full-length under the bivy. (I'm getting old and I'm discovering that I need the extra padding of the Prolite.) That's enough insulation for my legs, it adds R value in general, and it protects the bivy floor better than a polycro groundsheet would anyway. At a bit less than 3oz, yes, bringing the Thinlite totally negates the weight savings from the cuben floor, but in my case at least I'd be using it anyway. It seems like it would be handy with an inflatable like the NeoAir as well.

If you're talking about a 3oz difference not meaning much, well my homemade bivy weighs 3oz.

So you're saving a lot of weight by going with the Cuben.