Darn. I don't care if it's waterproof so much as it allows the frost to pass through my bag to the bivy, so the frost collects on the outside of the bivy. I'm not worried about getting rain or snowed on, I can carry a tarp for that.
I may have to check this out next time I'm at your house. The price is definitely right on these! Do they have zippers to get in, or do you have to climb in through the hole where the head is?
Basically, I want a sleeping solution that allows me to be able to sleep under the stars in the snow or cold and not have to worry about my bag being covered in ice or water droplets the next day. I imagine I need the bottom of the bivy to be waterproof enough that snow or rain doesn't soak through and get me wet from underneath. I don't need the top to be waterproof, I can use a tarp above me for rain/snow conditions (not too frequent here in Utah).
It's probably obvious that I still don't understand dew points or how at certain temperatures you can get condensation/ice in the middle of your bag, before it even reaches the outer shell. Davy (or anyone else) can you explain that to me?
Your bivy sack touching your sleeping bag will help it to move through to the outside. Most bivy sacks do touch.
On the dew thing. Your sleeping bag insulation gets progressively less warm as the water vapor moves from your body, through the insulation, to the outside of the bag. When it hits the point where the vapor cools down enough to turn back into a liquid, it condenses. Whatever layer it is at in your bag it stays there. As things cool down through the night the depth of "cold" can increase to closer to your body. The liquid water that was stuck there now gets cooled down to the freezing point and you get frost. This most often occurs on the shell of the bag or the inside of the bivy sack because it is place that is finally cold enough. In cases of extreme cold though it can happen in your insulation in your bag.