I leave the entrances WIDE OPEN, and jack the outer cover up about 1/2"
with sticks laid across the inner cover rim.  This allows air movement up
under the outer cover, and across the inner cover, with bee-escape hole
open.
 
Dry bees with lots of ventilation and plenty of food can  keep themselves
warm enough.  Soggy bees will die.  That's why packing them up too much can
have detrimental effects.  Please note the following illustration:
 
Many years ago, Dr. C. L. Farrar at the Univ of Minnesota overwintered a
colony in two deeps (right through a typically frigid Minnesota winter),
which went on to produce an excellent above-average crop the next summer.
Not a big deal,
except for the fact that the hive's sides were largely composed of nothing
but window screen!  Really!  He wrote it up in the bee journals and
provided photograhs to prove it!  Bees CAN survive the cold if they can
keep DRY, even if
that means some drafts.  If, on the other hand, the moisture builds up
inside the hive due to inadequate ventilation, then they are likely to
perish, because that moisture has nowhere to go.  It turns to frost in the
hive, or worse, condenses and drips back down on the cluster.  The combs
get all moldy - it can be a real mess.
 
When I was a kid out near Boston there was a big old tree in our
neighborhood that housed a thriving bee colony.  The gaping hole in the
side of the trunk was at least a foot long, and perhaps 9" across, up
around 15 feet from the ground.  If you looked up into that big hole you
could see the bottoms of the bees' combs.  That colony just went on and on
-- they consistently survived year after year, even with that seemingly
extreme exposure.   Right through the cold New England winters, and even
with the neighborhood kids' occasional attempts at throwing rocks and
sticks up into the cavity!  I think that colony was also a good example to
show that the bees can take ventilation without harm.  Note, too that it
has been found that the bees do not try to heat the interior of the hive or
cavity, but rather the interior of the cluster. The outer layers of bees
actually act as the insulation for the center.  A short distance away from
the outermost layer, the temperature is about the same as it is outside.
(C. L. Farrar did the research on this as well, back in 40's I believe.)
 
I often place a 2"-high rim under the inner cover on the hive.  It has 3-4
half-inch diameter holes across the front.  This rim gives the bees lots of
room to migrate over the frames to keep in touch with food.  Also, it
provides space for feeding dry sugar (the original reason for the rims).
The bees do well with these vent holes, and use them often for cleansing
flights.  With the lower entrances open, and the covers jacked up, I hardly
ever have moisture accumulation in the hives.  The bees do consistently
well with this arrangement.