Rectangular sleeping bags are heavier and bulkier due to its lack of tapered ends. It is also inefficient to heat insulation since there is more space for your body heat to heat up. On the positive side there is more room to change sleeping positions and moreover they can be unzipped and used as a blanket. Mummy Style sleeping bags feature a tapered cut that increases heat-trapping efficiency. This allows you to choose a lighter and less bulky bag and still get comparable insulation. These are your best choice for colder, high-elevation conditions. Semi rectangular bags can be used for both camping and backpacking. They sport a tapered design and they offer greater warmth and efficiency than rectangular bags. They also provide plenty of roomy for a comfortable sleep. They are especially popular with larger frame backpackers or restless sleepers who don’t like the tight fit of a mummy bag. They are more spacious than a mummy bag and less bulky than a rectangular. If want you want maximum warmth, choose a mummy bag that has narrow shoulders and hips.