I experimented with a homemade Tyvek Bivy this sheep season. I was cold and wet every night, mostly due to sleeping on a glacier with an inadequate pad. However thats exactly why I wanted a bivy in the first place, to keep my bag dry in adverse conditions and maybe even camping out in an emergency. Has anyone ever found a bivy that kept their sleeping bag dry in all conditions (like pouring rain or wet snow) or is condensation a given? I mean right now I am thinking that a tarp is a lot drier than a bivy...thanks.