Why does a downfilled sleeping bag keep you warm on cold night why is it useless if down wet?

A down-filled sleeping bag keeps you warm on cold nights because down is a natural insulator that traps air and retains body heat. Down clusters have a three-dimensional structure that creates loft and traps air, forming a barrier against the cold.

When down gets wet, it loses its loft and insulating properties. Water molecules displace air from the down clusters, causing them to collapse and lose their ability to trap heat. This is why a down-filled sleeping bag is useless if it gets wet.

To prevent down from getting wet, it is important to use a waterproof or water-resistant sleeping bag cover or shell. This will help to keep the down dry and maintain its insulating properties. It is also important to avoid storing down sleeping bags in damp or humid environments, as this can cause the down to absorb moisture and lose its loft.