The unpleasant sweat on the outside of a toilet is condensation that forms when the cold water inside the tank chills the tank material in a warm bathroom. This condensation can ruin any cloth covering and can be so severe that it causes puddling that may ruin floor coverings, finishes or trim. If the problem is severe and a homeowner is willing to opt for a more expensive, sure fix, a plumber can add a mixing valve at the tank inlet to introduce warm water into the tank. However, another option involves a less expensive, do-it-yourself fix.
Empty the tank of water and let it dry thoroughly.
Cut sheets of polystyrene or foam rubber to fit the inside of the tank walls. Or, install a prefabricated tank liner (available for purchase from most hardware stores).
Glue the foam or liner in place with a silicone cement, following instructions and tips on the adhesive.
Let the adhesive dry for at least 24 hours.
Refill the tank.
If desired, add a cloth tank cover to insulate the tank.