Don Vandervort, Head Homeboy, has written more than 30 DIY home improvement books, been a segment host on HGTV, served as MSN.com's home improvement expert and written countless magazine articles.
In humid climates, warm room air can condense on the cooler surfaces of a toilet and drip onto the floor, encouraging mildew and maybe even rotting the subflooring.
Tank water that's cooler than the ambient air causes moisture to condense and the tank to sweat.
The simplest solution is to empty the water from the tank and glue a foam toilet tank liner on the tank's inner surfaces—this will reduce the amount of sweating.
For a more thorough and permanent solution, talk to a plumber about installing a tempering valve on the cold water line that supplies the toilet. This draws a little hot water from a hot water pipe and mixes it with the cold water entering the toilet tank to raise the tank's temperature.
