Moisture on the outside of a toilet tank is condensation from warm air forming on the water-cooled tank. To prevent the problem, insulate the inside tank walls. First, shut off the water supply, then drain and dry the inside of the tank. Cut ½-inch-thick foam rubber to line the insides and bottom of the tank. Don’t let the foam interfere with moving parts. Use silicone glue or rubber cement to hold the foam rubber in place. Do not refill the tank for 24 hours.
If the water company consistently delivers water at less than 50 degrees, you may have to install a tempering valve to mix hot water with the cold that flows into the tank.
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