> the bivvy doesn't breath at all, which means your sweat during the night
> will turn into condensation inside the bag. Depending on how much you
> sweat and how cold the temperature is, this might be an issue for some
> people. In my test it just meant changing my clothes.

Yeah, it'll be a problem for my wife and me. She covers her head up with the blankets when she's cold, and when we slept under a space blanket, the condensation soaked our sleeping bag. I don't cover my head, but I sweat a lot.

I'll stick with space blankets that we can get out from under to cut down on that condensation. We need breathable gear in our situation.

Situations vary, of course.