never tired this but some one once told me string up a cord with a couple light bulb sockets the heat from the bulbs will keep it dry like i said i never tried it
my metal de-tached garage used to do the same thing. finally insulated it and put a metal ceiling in it , creating an enclosed attic (non acessable). stopped about 98% (guessing). only time I get sweat on the stuff inside are extreme circumstances in weather changes, or heating it up too warm too fast. the foam board type insulation is cheap and works pretty well if you seal the seams. the thicker the better tho, and the foil faced is way better than non foil faced, but a lot higher,=$
