You are correct that there was no insulation on the bottom of the hive. Just the coreflute. There was not a suffocation problem because the snow never got deep enough to even reach the bottom board this winter. The lower entrance was open about 3" and I had the vivaldi board on top packed with burlap. There was air flow through the hive. When I did the check of the bees in December, the burlap was dry. The inner surface of the lid was totally dry. No signs of moisture or mildew anywhere.
I insulated the top and three sides, leaving the South side open to the sun (In the Northern Hemisphere, hives face South) I chose that insulation based on my local mentor who does basically the same system. The only difference is that he does not first wrap his hives with tar paper. I put the tar paper on as a secondary wind break because my hives are in the middle of a field. Also, the black paper absorbs more sunlight/heat on the exposed side to theoretically help warm the hive.
The bees were exploding, several frames of brood and laying queen in the last week of October. I was feeding fondant because they did not have a ton of food stored and with the amount of bees in the hive, I was worried they would run out of food. They were storing fondant and buzzing away as late as December 30. Then the cold snap hit. I didn’t open the hive for the month of January and the first 40+ºF (4ºC) day we got in February, I went up to have a look. That was February 18. The Flow Hive was gone.
During the search for the queen, I did see a fair amount of varroa (I picked through the bees close up for 45 minutes) but not an alarming amount. If this helps in any diagnosis, the thing I did notice was that the bees were very evenly distributed (dead) on the bottom screen. There’s wasn’t a deep pile near the entrance/exit, but a layer across the whole board. It seemed to me by the way the were dispersed that they died quickly, like there was a deep freeze and they all just fell off the frames?
Temps were in the -10s to -20 C at night for a couple weeks in January (not counting the wind chill.) Not super cold for bees from what I’ve read, but probably cold enough to chill a small cluster if they were already weakened by some other disease.
Saddest thing was all the honey stores left behind. They worked so hard all year!
I’ve ordered two new Russian Nucs for this season. They’re RHBA certified and locally bred just an hour from my house, so they are from my climate. I’m looking forward to getting to know some new bees.