As I see it there are 2 basic types of bivy bags. There is the kind that is designed to be used on its own as a tent (often with a hoop over the head), and there is the lighter weight kind that is made to be used in conjunction with a tarp and just keeps your bag dry. The one I have wieghs 8 oz and has a bug net, but i would not want to try using it without a tarp. It would do for a night outside in an emergency.
As regards a good bivy - for me a good bivy is one that breathes well. If I bivy does not breathe well then you end up with a wet bag anyway simply from perspiration. And that brings up another point - always try to breath out of bivy and sleeping bag.
I also did do an experiment about clothes in a bivy bag. I always used to be a firm believer in less clothes the better for sleeping. Anyway I took along a 40 degree down bag on a trip in May and a freak cold snap with temps down into the teens had me freezing at night. So I put on my puff pants and puff jacket and all my clothes on and still froze. So the next night rather than wearing the clothes I stuffed them in and around me inside the bag - basically putting the extra insolation where I needed it. And I slept toasty warm!!
Basically I think wearing the clothes might insulate you from the bag and not let you warm it up? I also think with the clothes around you lets the bivy and sleeping bag breathe better so the moisture does not build up. Anyway those are my thoughts on the matter. Patrick
