An internet search will find many people claiming that Humid Air (water vapor) rises. However, I am skeptical because if lighter molecules rose then, it follows that the air would be stratified by molecular weight.
Facts: Water vapor has molecular weight = 18 Nitrogen has molecular weight = 28 Oxygen has molecular weight = 32
So, Water Vapor has the least mass.
However, If lighter gases rose then (using the above data) Nitrogen would rise Oxygen would fall and we would be breathing 99% Oxygen (neglecting the 1% of other gasses) this is not true So I believe that gasses do not behave this way as illustrated by the fact that we breathe 78% nitrogen and 21% oxygen Therefore, it seems to me that (in a closed environment) gasses mix randomly, with perhaps a very small gravitational contribution.