In this area a common successful configuration for winter hive ventilation
comprises three elements: 1. A reduced entrance below.  2. An auger hole at
intermediate height.  3. A shavings box on top, with the top of the shavings
exposed to outside air under the outer cover.

The usual reason cited for this configuration is "upward ventilation", a
need mentioned by Langstroth, and many since.  I've noticed that very little
air filters through shavings, but the shavings are damp on top, and dry
below nearest the bees.  Similarly, a shavings pile in our yard is always
wettest just below the plastic covering on top, and wet shavings deep within
the pile can be dried by opening the cover on fair days and closing it at night.

Is the shaving box ventilating the hive, or is it really wicking out water,
which then either evaporates or sublimes from the top of the shavings, with
little actual passage of air?  Water vapor is lighter than air and would
collect in the top of the hive, where it might permeate the shavings.

Similarly, is the CO2 produced by the bees' metabolism, being heavier than
air, simply sinking and flowing out the entrance, replacement fresh air thus
being drawn in through the auger hole?

Bill Morong