1. Size matters. Climb in different models and check wiggle room. A superefficient mummy cut is no use if you can’t sleep.
2. Go long. For cold-weather bags, get the long size. The extra space at the foot is a great place to keep water, batteries, and electronics from freezing.
3. Dress right. When you wear excess layers to bed in a trim-fitting mummy bag, the insulation compresses so the bag can’t keep you warm. But if your bag is roomy, layer up so your clothes take up the dead air space.