We've been struggling to find a sensor that could handle occasional 100% humidity conditions in tropical caves. So far all of the digital sensors have failed in more or less the same way: they work OK for a couple of weeks, then suffer from rising readings for a week or so till they hit their maximum and just stay pegged there from then on. What I believe is happening is that the conditions go condensing, and then the moisture simply never evaporates again from inside the sensor material. After a few days "dry" the sensors return to normal operation. So if you could find a sensor with an internal heater & condensing environment reset procedure I'd turn that feature on every couple of days.
I also tried a Masons Hygrometer:
but the DS18b20 temp sensors did not have enough resolution to measure the 95-100% range that I was after. If you used higher bit depth temperature sensors you should be able to make this old school method work for your mushroom farm provided you can keep one of the sensors dry (say inside a housing?) and the other one in a wet sleeve.