The PCB was tested and can be ordered on OSH park.
Design files available in the files tab.
The code is simple. By default humidity measured by the probe, battery capacity and battery voltage are outputted each two hours.
Two prototypes are working actually. They are both running @GertSanders bootloader (8MHz internal and LED can be connected to D8).
The first prototype is working using 2 AA battery, the second is working under a CR2032.
Expected duration more than 2 years for the CR2032 and about 3.5 with the two AA (power consumption less than 5uA during sleep time; measured using a uCurrent Gold).