Carbon Dioxide is NOT heavier than air.  If it were, there would nothing near
the ground on earth but carbon dioxide expelled by animal breath.

The cluster of bees is WARM, and kept warm by eating honey.  Their breath is
warm and all "hot" air rises.  This is why a bee hive SHOULD ABSOLUTELY have
an UPPER
entrance to allow this moisture filled air to escape the colony rather than
condense into water droplets as it does in you shaving pile UNDER PLASTIC.
If you had an UPPER entrance in the plastic cover, shielded from rain by some
sort of umbrella, your shavings would be dry.

I am starting my 67th year of beekeeping in 2000.  I have NO holes drilled in
any body (that is stupid), I have no reducers inserted in the bottom board in
order to allow air movement into the colony; but I have a 1" slot cut in the
front edge of the INNER COVER as an UPPER ENTRANCE.  This "hole" lets the
rising moist air out in the winter, and the foraging bees use it in the
spring and summer to enter and exit the
supers without going through the congested brood chamber below.

You can read about many of my writings on

http://www.cybertours.com/~midnitebee/                click the lower right
image
or http://www.beekeeper.org/george_imirie/index.html

I hope I have helped.

George