I am located at the northern edge of the TN valley, with weather close to
that of KY (on a ridge top, we skip some cold snaps, but have all the rain
you could want in winter, very little snow usually and that for only a day
or so).  For ventilation, I run open bottom boards and an inner cover with a
ventilation slit (also covered in mesh) year 'round.  So far, no wet moldy
bees.  I still have two or three hives on solid bottom boards (and not many
total, to be sure), but they winter about the same as the ones with open
bottoms (so far, i lost one of each this winter).  My top covers are also
open in the front, allowing full ventilation from the open mesh section of
the inner cover.  With the exception of one hive, none have seemed to
complain about the ventilation (that one has covered their entire entrance
in propolis, except for two bee size holes, this winter).  I do know that in
the summer, switching a very strong hive from solid bottoms, a regular inner
cover and regular top cover to the above described setup results in huge
beards of bees going back into the hive (while a second strong hive next to
it stayed outside trying to stay cool for three more weeks). Since I haven't
seen a difference (in this climate) for wintering and the summer benefits
were so much greater (double the honey from the more ventilated one this
year), they are all being switched this coming season.

K Oland