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.
What was the temperature in the room ?