Toilet tanks usually sweat as a result of too much humidity in the bathroom. Bathrooms are usually the dampest room in a house, and they typically don’t have a lot of ventilation, especially if the room doesn’t have a ventilation fan or if the fan isn’t running frequently. To reduce the likelihood of toilet tank sweat, take steps to lower the humidity in the bathroom, including regularly running air conditioning units or dehumidifiers, which will help reduce condensation on the toilet tank. Learn about how to reduce toilet tank sweat and possible side effects, such as mold, mildew, and water damage.