A toilet gets the sweats when warm, humid air in the bathroom hits the cold surface of the tank. Hello, condensation. If this only happens once in a while, hey, no big deal. But moisture dripping from your toilet all summer—in cottage country, this can happen if you get your water from a deep, cold well—will eventually damage the bathroom floor.
Reducing humidity and moisture in the bathroom—by running an exhaust fan, and wiping down wet walls after you shower, say—can help. And as a cheap and temporary solution, you can install a drip tray under the toilet tank. It’s not beautiful, but it’ll protect your floor, just as a coaster protects your table from a sweaty drink.
Meanwhile, check the toilet’s flapper. If it isn’t sealing properly, water will leak from the tank into the bowl, and the tank will keep refilling itself with cold water (a.k.a. “ghost flushing”). In this case, the toilet bowl will likely sweat too. Either replace the flapper if it’s damaged or is the incorrect size, or adjust the chain that connects the flapper to the flush lever.
Inspect the tank’s insulation lining. (It keeps cold water from directly touching the inside walls of the tank.) It can degrade over time, and may need replacing if it looks cracked or brittle. Or, the tank may not be insulated at all. Either way, you can buy liner kits from building-supply stores, and insulate the tank yourself.
No joy yet? Try installing an anti-condensation mixing valve. It’s designed to add a small amount of hot water to the tank and raise the overall water temperature inside.