Stop a toilet from sweating by decreasing the humidity in the bathroom or by increasing the water temperature in the tank. Water from warm, humid air condenses when it comes in contact with a cold tank. While toilet tank insulation kits claim to help, they are often ineffective.
The options for decreasing humidity in the bathroom include using a vent to remove warm moist air from the room or using air conditioning. Operating a vent is usually less costly than operating an air conditioner. However, to keep humidity at a level where the tank does not sweat involves diligence in operating the fan each time someone uses water in the bathroom. Lowering the relative humidity in the bathroom means there is less water to condense on the tank.
Increase the temperature of the water in the toilet tank to that of the surrounding air by installing a mixing valve. The valve mixes the needed amount of hot water with the cold water entering the inlet so that the tank is the same temperature as the bathroom and does not sweat.
Installing the valve involves cutting the water supply line to the toilet and tapping into a hot water supply line. Homeowners who are not comfortable with the installation should call a plumber to install it.