What’s the lightest, most breathable shell there is?

Nope, it's the cuben craze as i call it.

Cuben fiber isn't actually the best material for ponchos, rains skirts, or anything that will be subject to abrasion, because unless it's fortified with extra polyester or nylon material, it will abrade easily and more quickly than many other woven fabrics.

Silnylon on the other hand is excellent for the above, because woven nylon is tough as nails to begin with, but add a coating of slick, slippery silicone, well you got something that tends to glide off things. Sure, some of the silicone will eventually come or wear off, but easy to replace and meanwhile will likely go longer with the same amount of abrasion and still be waterproof as compared to cuben.

When i realized the above, i sold my cuben poncho tarp, because i'm looking for long term durability (and i realized since i like winter backpacking the most, tarps aren't for me so much).

Tarps and tents with stronger cuben, great stuff. They are not usually subjected to much abrasion wear though.

So, to me, it makes sense to go with the silnylon over the cuben for a poncho unless you are Ian money banks who doesn't have to worry about money and purchases at all. The nice thing about the cuben poncho though, is that it won't get soggy with moisture as can eventually happen to the silnylon poncho during a heavier or prolonged rain.

Ian – It is the dual use. I have a Zpacks Hexamid Twin and the Zpacks poncho/groundsheet clips right in and forms a twin sized bathtub groundsheet. So I save at least 3.5 ounces over having to bring a twin groundsheet, not to mention the money I save not having to buy one. So with the dual use in mind, knock over a hundred bucks and 3.5 ounces or so off the Zpacks poncho/groundsheet and it nets out very well compared to sil nylon stuff. I prefer bathtub groundsheets over the window film stuff as a personal preference. The rest is just sad cuben envy….

Edward, sounds like your set up works well for you and i like the combo/dual use aspect.

re: cuben envy. Well, not so much, i had a custom zpacks cuben poncho tarp, but sold it because i realized cuben + poncho isn't the greatest combo for maximum durability (which is important to me for various reasons, but can be offset by comfort factors some depending) and because i wanted a more closed up shelter for winter backpacking.

But i do have a cuben solomid. I have enough cuben material to make myself a cuben rain kilt (patterned after a kilt, so i call it a kilt), but again, don't see the point so much–might make it and sell it for reduced price since i'm not using that extra cuben.

So no, cuben envy not so much. Every material has it's strengths and weaknesses, it's best application for this or that area. Unless we're talking the hybrid cubens, then abrasion resistance and application where same is likely to happen, is just not a strength of cuben fiber. This is actually it's greatest weakness.

The Zpacks Poncho/Groundsheet is great, if it fits you. I'm a relatively slim build at 5'10 and 175lbs and I just couldn't get it to work with me and my pack. I just couldn't be as mobile as I like to be in alpine environments. I suppose if I was only walking along well-groomed trails, I could make it work though. The fit problems I had were mostly about the upper body and shoulders area. I can see why it works for a guy like Joe.