Hey Sky, I should have said I already have a (expensive for what it is) Sentinel environmental controller. I would love to switch to a Pi when funds allow. I have temp and rH perfect after a week of fiddling. My A/C doesn't have a thermostat readout; it's just a knob.
Adding CO2 is my next goal. Those controllers are pricey too. Have you worked on that? I know OpenAG uses off the shelf CO2 sensors, and I'm sure the code is available.