OK, if ya’ll want a thread to debate wintering I’ll start a new one. I don’t care if you go off topic or not; that always happens anyway.
First off, T Beek is absolutely right to be concerned about insulated hives turning into refrigerators. I have seen that happen on numerous hives and nucs in my bee yard this winter. Insulation is only a positive thing if the box has a lot of bees in it. If the bee numbers have fallen too low (due to varroa losses, or other losses) an insulated hive will act as a freezer. There is NO DOUBT about that.
I had 25mm thick foam hives with ice in them during our last warm up to 44F (6C). They were ice cold inside. That happened because the number of bees in the box had fallen too low and the foam keeps out any solar gains. I had other 25mm thick foam boxes with more bees that were toasty warm. The only variable between ice cold and toasty warm is how many bees you have. Too few bees and you might as well winter them in a freezer. :(
Once an insulated hive gets cold, it’s a losing battle for the bees because of the heat capacity (thermal mass) of all that ice cold honey. It takes too much energy to unfreeze all that honey. The 10 to 20 watts of heat the bees can generate aren’t going to cut it. You either need solar gain (like a wrapped wood hive) or electric heat. On a sunny day, the heat from the sun on the surface of a wrapped bee hive is WAY more watts of energy than what the bees can ever generate. Hundreds of watts vs tens of watts.
I’m sticking with 38mm thick foam hives for my bees. Done right 38mm foam works very well in Michigan if your hives are full of bees. 25mm foam is just too thin for some of my nucs in Michigan. I’ve got 4 frame medium nucs in 38mm of foam that are still doing fine while a lot of my 6 frame 25mm nucs froze during one of our bitter cold nights (0F/-17C).
The way to tell if your insulation is acting like a freezer or an oven is to check the temperature inside. The temp inside a foam hive with a sufficient ball of bees will be toasty warm. If it’s not toasty warm, you have good reason to be worried. :(
Let the flaming begin ;)