My toilet is getting a great deal of condensation on the tank and dripping onto the floor. What can be done to address condensation on the exterior of a toilet tank?
When warm moist air comes in contact with a cold surface condensation forms.
There used to be these tank liners sold that you would install on the inside of the tank to insulate it. It keeps the outside of the tank from getting as cold to reduce or eliminate condensation. I did a quick search and couldn't find it anymore.
I did however find toilet tanks that come with insulation already installed. See http://www.homedepot.com/p/KOHLER-Memoirs-Classic-1-28-GPF-Toilet-Tank-Only-with-Insuliner-Tank-Only-Liner-in-White-K-4433-U-0/203006152#
The liners were just sheets of styrofoam. I guess you could get some styrofoam and cut it to fit. You'd have to cut the kits anyway. Then glue it to the side and bottom of the tank. Going with a tank that's already insulated would be a lot easier.
While it is not a simple solution, one method of ensuring that toilet tanks do not sweat is the use of a mixing valve on the water feed to the tank.
These valves mix hot with cold water to feed warm water to the tank. Warm water prevents condensation. The valve only needs to be set once unless you significantly change the temperature of your water heater.
This is usually done when the toilet is being installed, but it can be retrofitted if you can get access water lines and there is also a hot water line near the riser to the toilet.
There are at least four strategies:
Decrease the relative humidity with a dehumidifier in the room. Also, heating the air around the toilet decreases the air's relative humidity while simultaneously increasing the tank's temperature.
Strategy #2 could be done with @bib's solution for heating the water. Or you could skip the mixer and run straight hot water to the tank (with no cold feed). Or you could rig some kind of heater in the tank. An electric solution seems replete with concerns but maybe a circulating radiant system exists at the location?
@OrganicLawnDIY's answer is one way of implementing strategy #3. Another method would be to insulate the outside of the tank.