Control crawlspace fan based on humidity?

Unless things have changed, there is a smart app in the marketplace section of the mobile app under climate control called “smart humidifier” which will let you do exactly this. You just check one sensor for humidity and then you can turn on or off any device which presents as a switch. So that would include the smart pocketsocket that you have the fan plugged into. Or If you need a more complicated rule structure, you can always use webcore.

As far as a good humidity sensor, depends how accurate you want it to be. Many of the inexpensive open/close sensors also have humidity readings. So do most of the multi sensors. Or you can go to something specifically made for that purpose which will be more accurate, but also more expensive.