[aquila48]

I actually did an experiment using two boxes whose tops are covered with cellophane (simulating 2 small greenhouses). I filled one with air the other with CO2 (100%). After many days in the same room, there was no difference in temperatures within the accuracy of the thermometers.

If CO2 absorbed infrared radiations significantly more than air, then the CO2 greenhouse should show a higher temperature. (The way a black surface absorbs more sun light and gets hotter than a white surface).

The most likely reason it doesn't show any significant difference is because the peak absorption area for CO2 occurs at a wavelengths outside the area where most terrestrial radiation is emitted. See figure below.

[UCANSEE2]

After many days in the same room, there was no difference in temperatures within the accuracy of the thermometers.

What was the temperature in the room ?