I have a complete arduino based domotica system up and running.
Now I would like to add a CO2 sensor to some rooms to control the air quality.
I already tried the MG811 modules by sainsmart but even with an opamp they do not seem to react to my breathing on them, let alone be able to measure CO2 levels in a room.
I have a feeling that when they start to work I'd be long dead
Looking at the datasheet (specifically the graph there with Sensitivity) it looks like it isn't much good below about 400ppm. Normal atmospheric CO2 is about that or maybe a little lower. So you're operating right on the edge of what that sensor is capable of unless you have significantly increased CO2 in the room.