I was reading Randy's article in ABJ this month at lunch.   In it he seems to take a bit of exception to upper entrances and moisture.  For wrapped hives in most areas  I would say they are critical.  He points out that moisture condenses on the sides.  Correct,  but a upper entrance allows for heat convection to pull in dry air and allow wet air out.  
Having wintered bees in Northern IA,  and here in southern IL,  I can tell you there are huge differences.  In IA the cold DRY air set in mid November,  and wintering bees was actually a bit easier.  Cold DRY air helps,  but moist wet air  like we have here in mid southern IL is a nightmare.   You wrap a hive here without a upper entrance and you will have a dead wet moldy mess in spring. 
  
I did use upper entrances in IA,  smaller than I use here,  but still present.  This allowed a small amount of "chimney" effect in the hive,  and you could normally see cold air  frost around the upper openings.  Here in IL I use a 3/4 X 4  upper and a 2" lower opening.  

Your relative amount of Cold and dry is a key component of this issue,  but if you don't have air exchange in a tightly wrapped hive,  you will have problems. 
 
For several years I used some BeeMax  foam hives.   Didn't notice any help with them until I punched a 3/4 hole just below the top. Of the box.  This lets a bit of air exchange, and wintering success went up dramatically in the foam hives from that little tweak.


Charles

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