Joseph Meehan wrote:

...is it not likely that in the confined space of a window would diffusion
not even out the moisture content before it would become a serious factor
in air movement?

Convection is usually more important. IIRC, Fick's law says 0.2RhoAdw/dx g/s
of water vapor would move through area A cm^2 with a dw/dx absolute humidity
gradient normal to A, where Rho is water vapor density in g/cm^3, but there's
still a bouyant force from humidity if the average water vapor density in
the cavity is less than the density in the air surrounding it, whether the
density in the cavity is uniform or not.

There are times when holes in the cavity top won't help, eg when it's close
to 100% RH indoors and outdoors at the same temp and one temp drops, which
might cause temporary fogging, or when it's very humid outdoors for a long
time in summertime with house AC and the window cavity leaks air out at the
bottom which gets replaced with warm humid air from the top holes. That's
when venting to the indoors would help.

Nick