When a cold surface comes in contact with warmer moist air, moisture will condense on the colder surface. Such condensation will cause a toilet tank to sweat. Over time, this moisture can rot the floor. Insulation kits are available to prevent this. They require that the tank be flushed and dried.
Test-fit the insulation and then apply silicone caulk or the adhesive recommended by the manufacturer to the back of the panels. Insert the panels in the tank, applying pressure as you fit them. Let the adhesive cure completely. Turn the water on, and adjust the float valve if necessary.
An antisweat valve mixes a small amount of hot water with the cold water flowing into the tank, raising the tank temperature just enough to keep moisture from condensing on it. Some valves have a manual control to vary the mixture for different humidity levels.
Shut off the water to the house. Following the manufacturer's instructions, cut the water lines to the bathroom. Cut pipe to fit your installation. Dry-fit the pipe before soldering the joints, making sure everything fits perfectly. Keeping all the parts at the correct angles, presolder any joints that will lie too close to the joists when installed and might pose a fire hazard if soldered in place.