What is a bivvy bag? Sometimes called Bivi bags or Bivouac sacks, these are waterproof enclosures which surround your sleeping bag, forming a cocoon of protection against the elements. Originally invented by climbers who needed emergency weather protection for their sleeping bags during multiple-day ascents of mountain peaks – the bags had to be lightweight and easily packable in a small climbing backpack. Today’s designs have more in common with tents in terms of the…