You can minimize the effects of humidity on down by 1) squeezing all of the air out of the bag as soon as you get out of it in the morning. This gets rid of whatever water vapor that has accumulated in the air spaces from your overnight perspiration; and 2) Airing and sunning the bag whenever you get a chance during the day. Hot sun will drive out a lot of the moisture that remains after you give your bag it's morning hug. Keeping the bag away from rain and wet tent floors is always a good idea too. It is easier to keep a down bag dry than it is to dry one after it gets wet.