Nope, that's not a mummy
Posted by: Waterbird on Mar-12-13 11:40 PM (EST)
The fact that a sleeping bag has a hood doesn't make it a mummy. "Mummy" refers to the tapered foot and being form-fitting throughout. Quite a few rectangular bags have a hood.
Big Agnes sleeping bags are huge rectangular bags. However, they are made specifically for people who sleep on their back. There is no insulation on the bottom bag. You insert your sleeping pad into a pocket on the bottom of the bag. The result is that there is only one sleeping position that really works: on your back.
Semi-rectangular is a very comfortable shape. It allows full movemet but reduces the huge air space that you have to warm up around your feet. At a temperature of 20 degrees, that air space is a significant concern.