The bivi bag is made without a zipper. It is made with snaps that off set those on the sleeping bag. One side is the male and the other female snap. In this manner the bivi bag can be closed if used by itself. There is a generous flap that covers the center of the bivi to keep out rain. The bivi connects to the sleeping bag with the snaps and does not impede the opening of the bag in a hurry if that becomes necessary.
First impression was the light weight nylon as used in current ponchos. The SNAPS required a little TLC on a few that were very hard to open without a metal shim to pry apart. I dressed all with beeswax and all is good. Go slow the first time with them. The flap is a true 5" and overall fit loose but not sloppy. If you are O.K. with the bag this will not decrease movement. It came with a 6.5"x13" stuff sack same material. It just as easily fits into the main stuff sack with bag inside it and you have a nice bag for other gear.
We are in a high wind advisory and overnight freeze alert for Southern California. NO, you do not get 10 degrees+ insulation as others falsely state. You WILL NOT LOSE any warmth to windchill I confirmed outside; no thermometers except the one in my head. I believe the centerzip will allow venting if sweat condensation becomes critical.
– Christopher Kavanaugh