Why use sil-nylon on the bottom of a bivy?

Am I missing something? Do we really need a waterproof floor on a bivy for 3 season?

(chuckle) Well, being that Ryan's locale is "25% Wyoming, 75% AZ", I suspect he doesn't need a waterproof floor. But yes, he's missing something … frequent measurable precip and soils that retain moisture. It's another classic YMMV question/answer.

>frequent measurable precip and soils that retain moisture. It's another classic YMMV question/answer.<

Clearly a YMMV issue. I guess I ask b/c I have camped all over the country and haven't found an absolute need for it, though I do see a slight increased functionality for negligible weight gain.

This summer I used my uncoated bivy in the cascades and slept a couple nights on saturated ground. I'm talking so wet it squished under my feet when walking on it. The uncoated nylon did get wet, but the water never soaked through my sleeping pad and the uncoated nylon dried very quickly (before packing, w/ a quick start) the next morning.

I polled a half dozen very experienced hikers today and the only reasons folks could come up with for wanting complete waterproofness on the bottom of a bivy were extended hiking in the rain where things wont dry (Olympics etc.) maybe on snow, though there were mixed opinions there, and then on slick rock in a heavy rain. Some folks said they would consider it on tundra too.

I can build a non water proof bivy at about 4oz and I think a water proof floor design for 6oz (I hope) not a huge difference, but every little bit counts. I think at this point I will likely build both and see which I use more. I can definitely see some advantage in some conditions for the water proof floor, but I have to ask if the performance to weight ratio is worth it. Like all things it is a judgment call for sure.

Here's the question as I posed it to folks:
You have a light weight sleep system consisting of a down quilt, an inflatable torso/3 quarter length pad, tarp and backpack under your feet and a bivy. Does your bivy need a waterproof floor?

Just thought it was interesting that every manufacturer makes 'em w/ a waterproof floor. I guess it is a "most useful for the most folks" thing.

Actually, it's a good question. If you use only your bivvy without the extra ground sheet, perhaps a waterproof floor makes sense, but why would you need it if you are also using a waterproof ground sheet, like polycro?

My one attempt at using a bivy (inside a single-wall tent in an attempt to supplement the sleeping bag in cold weather) didn't work out very well. The silnylon bottom of the bivy was so slick I was sliding all over! The bivy wasn't thick enough to use my air pad inside, so I was mostly sliding off the pad. After an hour or so of this, I removed the bivy. It appears you'd have to "paint" non-skid stripes both inside and outside the silnylon bivy bottom.

I'm just wondering how others cope with this situation?

Ryan and I were talking about it earlier today and I think the answer we're coming to is that we'll make both. One that is purely ripstop and one that is sil bottomed and ripstop top. That'll mean that we've got a quiver of options for differing conditions and shelter choices, because much like Mary D I believe the protection from convective heat loss that the bivy provides is worth it with a quilt system. Since we're making them ourselves we should be easily be able to do one of each kind of bivy for less than the cheaper yet quality commercial bivys (such as the Ti-Goat or MLD bivys).