When the water's really cold, the tank perspires
Our water comes from a well and enters the house at about 45° F. Last summer, that caused rather severe condensation on our toilet tanks. I've checked indoor humidity levels and they're not excessive. Is there anything we can do to prevent our tanks from sweating?
— Clyde, Granby, CO
Richard Trethewey replies: The same thing happens to a glass of iced tea at a summer picnic. Humid air hits the cold glass, cools, and gives up some of its moisture, which forms beads of water on the outside of the glass. You have a couple of choices to prevent this problem from recurring. Insulating kits — pieces of rigid foam that fit inside the tank — keep the cold water from touching the tank walls. They're available at hardware stores and home centers. While the insulation is unobtrusive, it can be a headache to install; tank shapes aren't standard, and you'll want to make sure the foam doesn't interfere with the toilet's float mechanism. Once in place, the kit is reasonably effective as long as the joints between the foam pieces are tight. It's also possible to buy a new toilet with an insulated tank, which will perform better than a retrofitted one. But in both cases, your toilet can still sweat if the incoming water is very cold and the room humidity is very high.
The most effective (and expensive) approach is to install a mixing valve that's connected to a hot-water pipe. Warming the incoming water slightly stops condensation completely, but there are some downsides. For one thing, you'll be flushing water you paid dearly to heat down the drain. For another, it's often difficult to install the valve in a location that only affects the toilet — you don't want warm water coming out of cold-water faucets, too. These valves are much easier to install during new construction.
Of course, you could decide just to throw a couple of towels under the tank to soak up the drips when the weather turns humid.