Condensation on a toilet can be so severe that you may think it's sprung a leak, and the water can seep through the floor and cause damage. First try insulating the tank. If that doesn't work (or if insulation won't fit), have your plumber install a tempering valve and turn it on during humid months.
Drain the tank. Turn off the water at the shutoff valve, remove the tank cover, flush the toilet and hold open the tank stopper or flapper to drain as much water as possible. Use a large sponge to remove any remaining water, and dry the tank with a towel.
Use a tape measure to get the dimensions of the tank walls, then use a utility knife to cut appropriate-size sections of foam from a tank-insulating kit. Test-fit the sections and spread adhesive on the tank walls to adhere the foam as directed (see illustration).
Have your plumber install a tempering (mixing) valve between the cold-water pipe and the shutoff, and extend a hot-water line from under a sink to the valve. Install a separate shutoff on the hot-water line to the tempering valve so you can turn it off when it's not needed.