If you're interested in a hardware solution, I've used the MH-Z19 CO2 sensor to great results. It's an NDIR (read:not electrochemical) sensor with a UART interface and temperature compensation. It will report up to 5000ppm and comes factory calibrated.

It's also $20 in singles from China. Coupled with whatever microcontroller you want, it's totally possible to have a distributed net of CO2 sensors for a low cost per node.

That being said, it's a ton of work and this is an excellent solution with much quicker results.