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How does a synthetic overbag work?

In really cold temperatures, it's often recommended to use a separate overbag made from a synthetic material over your main down sleeping bag. Supposedly this helps moisture (from your body) escape your inner down bag, keeping it dry and lofty. The escaped moisture should then freeze within the overbag's insulation, but at least it's not freezing in the down, degrading it's insulation.

# How does this actually work?

Additionally:

- Does the frozen moisture ever escape the bag(s) completely, e.g. through sublimation?