Those are not bivy sacs they are tents my friend. I have used several gore tex bivy sacs as protection for high end $1,000.00 down sleeping bags. I can say, I would never desire to use one for a shelter, only in extreme emergency. You will not escape severe weather in a bivy. Check out a North Face Mountain 25 tent - not the VE 25. It is a fantastic one person tent that is four season and will withstand anything including Everest type wind/weather. It's light and will get you out of elements. For decent weather in the hardwoods a bivy may suffice but if you get stuck on a mountain or severe rain/wind you will be dang unhappy with what a bivy does for you.