What is the number one drawback of using down in a sleeping bag?

The primary drawback of using down in a sleeping bag is that it loses insulation when wet. Down feathers provide excellent warmth and compressibility; however, when they come into contact with moisture, they can clump together, which drastically reduces their insulation properties. This means that in wet conditions, down sleeping bags may not provide the necessary warmth, and can take a long time to dry out once wet, making them less reliable for wet weather scenarios compared to synthetic insulation options.

While cost is a factor for some, it is not the most critical drawback compared to the impact of moisture on down insulation. The weight and bulkiness of down bags can also be concerns for certain users, but they are generally known for being lightweight and packable, especially compared to synthetic alternatives. Therefore, the risk of losing insulation in wet conditions is a critical factor that can significantly affect the effectiveness and reliability of a down sleeping bag in real-world scenarios.