CO2 as insulating gas in the window?

Hello everyone,

thanks in advance for the help and I hope I am in the right subforum with my question. Since I want to run ventilation through a window, I made a double pane out of Plexiglas (with two openings for the ventilation pipes), which I insert into the frame in place of the window. However, in the meantime, I realized that this is probably not ideal in terms of insulation. Therefore, I thought about whether a gas filling would help a bit (so far only air, dehumidified by silica gel in the PVC profile between the panes). Argon would be relatively inexpensive to obtain in small quantities, but even cheaper would be carbon dioxide from the soda water maker in the kitchen. Unfortunately, I have found nowhere why CO2 is not used as a filling in windows, although it has a similar thermal conductivity to argon? Would that make sense, and would the gas (no matter which) even be retained— the panes are sealed with silicone?

Would that make sense, and would the gas (no matter which) even be retained – the panes are sealed with silicone?

First of all, a very big question arises for me. How do you get the gas into the pane without "normal" air being inside? I would carry out the ventilation by means of core drilling next to the window (if there is space). I strongly assume that as a layperson you will never achieve a reasonable window at the level of a normal thermal insulation glazing.

A key factor in the panes is the thermal insulation glazing in today's windows compared to the insulating glazing that was installed until the mid-90s. A glass pane is coated with a layer that reflects infrared radiation. This reduces the U-value from 3 to 1. The gas is only secondary or only relevant for the first decimal place. You cannot achieve that with Plexiglas.

Thank you for the answers. I am already aware that I cannot compete with current thermal insulation glazing. However, the benchmark would rather be the installed windows. And since the house was built in the mid-nineties, I am honestly not sure if thermal insulation glass was already installed here. The lighter test was not really conclusive... I imagined the filling approximately like this: drill a small hole each at the bottom and top, fill with gas from below (or flush long enough), and seal both holes with silicone (?)