Things You'll Need
Toilet insulation kit
If you notice water collecting on the floor beneath the toilet tank, you might discover that it's not from a leak, but from condensation on the outside of the tank. This condensation forms when the cold water that refills a tank causes the tank surface to be cold. When hot, humid air -- such as during the summer months or long showers -- comes into contact with the cold tank, the tank "sweats." This water that sweats down onto the floor can lead to mold and mildew. Consider insulating the toilet tank to keep it from sweating.
Locate the water shut-off valve, usually behind the toilet's left side, Turn the valve clockwise all the way to shut off the water supply to the toilet.
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Remove the tank lid and place it gently out of the way. Depress the toilet handle to flush the toilet. Continue holding the handle down to empty as much water from the tank as possible.
Vacuum out the residual tank water, using a wet/dry vacuum. Wipe the tank's interior walls and bottom with a dry towel.
Measure the height and length of each inside tank wall with a tape measure. Cut the foam or rubber insulation material from the toilet insulation kit to fit the dimensions of each wall, using a utility knife.
Apply adhesive from the kit to the inside walls of the toilet. Fit the panels of insulation against the walls, pressing it into place. Allow the insulation adhesive to dry according to the manufacturer's instructions.
Turn the water shut-off valve counterclockwise to allow water to refill the tank. Place the lid onto the toilet tank.
Insulate a toilet tank if you notice constant sweating, because the water dripping onto the floor could eventually lead to rotting of the subfloor.