Mine has never gotten wet except when being washed in the 37 years that I've used it. But just condensation over a period of a few days could reduce the insulation considerably. When camping in freezing temperatures I use a vapor barrier liner inside the bag which lets me use the bag at well under its normal rated temperature and also almost eliminates condensation. In warmer conditions I let the bag air out by fluffing it up inside the tent shortly after arriving at the campsite so any moisture can dry out before I go to bed.