I really did not mean to make it such a difficult question. We occasionally will have a family "camporee" in late fall, in farm country where it is perfectly flat, no trees or windbreaks and on a clear night with no wind all the ground heat quickly radiates away. When there is no breeze at all, campfire smokes rises straight up 50-60 feet and it is almost eerily calm. Everyone gets a heavy coat of frost inside and out. I have a mesh tent with a fly, the fly will be covered inside and out and on a few occasions the mesh as well. Then when you move around, big sheets of frost will plop down on you and instantly melt. So I was asking not about waterproofing, but if it was possible to add some water-resistance to make the water bead up and roll off.

I don't even have a down bag. But every sentence where somebody writes about down, they also say "don't get it wet". Well, how wet is wet? Throwing it in the creek, sure. Slop a little water on it while getting a drink? Dew? A little rain comes in when you're making a potty dash? This sort of thing.