I highly recommend the Bibler Winter Bivy Sack...it is the ultralight hiker's dream (9 ounces, verfified on my scale)! I broke the bivy in during a hike through the White Mountains, NH in miserable conditions (sleet, snow, freezing rain). Paired with my SilPoncho, I spent the night warm and dry. The bivy did a great job of keeping my bag dry from any splashing rain, but I did find that I had to open the zipper on occasion to relieve some of the condensation. Despite this, I found the Epic shell to be quite breathable (C'mon, I have yet to experience any "waterproof/breathable" piece of gear that didn't have at least a little bit of a condensation issue...so I'm realistic in my expectations). This product gets my highest endorsement for the ultralight hiker.