Good point on the diffusion of CO2 from the bees through the top, for any
CO2  in solution in the air inside the hive.  Any CO2 in excess of that
might come out of solution.  I have no data CO2 solubility in air at any
temperature.  As to what floats and what sinks, according to the CRC
Handbook, at 0 degrees C, air density is 1.17 g/l, and of CO2, 1.97 g/l.
Were the CO2 to be warmed to +40 C its density would drop to about 273/313
times 1.97, or 1.72 g/l.  Were the air to be cooled to -40 C, its density
would rise to 273/233 times 1.17, or about 1.37 g/l.  So undissolved CO2
probably sinks at all reasonable temperatures.  This based on the gas law,
PV=nRT, and is approximate because the gases are probably not perfect.

So it seems that dissolved CO2 will diffuse any way it can, and any excess
falls down and goes out by gravity.  What comforts me, I hope justifiably,
is that with the right configuration the removal of gaseous waste is
passive, and conceivably done with little heat loss.  All this leaves me
pondering the idea that started the thought, whether there is an optimal
height for locating the auger holes I am about to bore.

Bill Morong