Marmot Alpinist Bivy A bivouac sack for the twenty-first century, the Marmot Alpinist Bivy is 100% seam taped for waterproofness and durability, weighs in at under a pound, and compresses like magic into any pack. High-tech Marmot MemBrain fabric breathes easily while blocking wind and water: The Alpinist Bivy offers awesome minimalist protection for climbers and backc ...