When cold water enters a water closet tank, it may chill the tank
enough to cause "sweating" (condensation of atmospheric moisture on the
outer surface of the tank). This can be prevented by insulating the
tank to keep the temperature of the outer surface above the dew point
temperature of surrounding air. Insulating jackets or liners that fit
inside water-closet tanks and serve to keep the outer surface warm are
available from plumbing-supply dealers.