I don't think a pillow case makes an ideal storage bag for a sleeping bag.

I've got two down bags that are more than thirty years old and a fairly new, very pricey summer down bag. The older ones have lost plenty of loft but are still quite usable. I have always stored them loosely folded, and only rarely washed them.

In South Florida, I'd quite concerned about heat and humidity and without climate-controlled storage, would probably put seal item in a very very large plastic bag preferably on a day of low humidity or after a brief period in a clothes dryer.

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