What is a disadvantage of a rectangular sleeping bag compared to a mummy bag?

A rectangular sleeping bag's design inherently affects its thermal efficiency compared to a mummy bag. Mummy bags are specifically shaped to closely conform to the user's body, which reduces the amount of air inside the bag that needs to be heated. This snug fit minimizes thermal loss, as less warm air can escape, keeping the user warmer.

Conversely, rectangular sleeping bags allow for more space, which can be comfortable, but this extra room means more air that needs to be heated. As a result, rectangular bags can be less thermally efficient, especially in colder conditions, because they do not trap heat as effectively as a mummy bag. Therefore, the correct answer highlights a key disadvantage of the rectangular design in terms of warmth retention and overall thermal performance.