Nice howto! Just wanted to mention, the humidity sensor MAY show a significant offset when exposed to high humidity for long periods of time. It's a feature of almost all capacitive humidity sensors and especially these cheap ones. I have expensive Sensirion sensors (DHT22 is a clone I believe) and even they have at least +/- 5%RH accuracy in high humidity + the offset of 3%RH or so after some time in high humidity. Of course in a terrarium +/- 5%RH is enough. You could have another system to check the sensor in the terrarium for offset every now and then.
I myself have not used Pi for my terrarium. I have a similar system, for temperature monitoring only at the moment. I am building RH meters which use the Sensirion sensors (high accuracy) and some day logging the humidity too. My main problem is the humidity controller I use. It develops a 10-15%RH offset which is a LOT. I also have a heater system which is controlled by an industrial PID controller.