Been doing a lot of viewing online books, videos, U-tube, forums, and talking to many folks. I am well aware this may get a bunch of hackles raised here and do not want a war. (I was at one time a residential/retrofit insulation contractor so I have than knowledge to help me)
First let me begin by stating that wood has an R-value of 1 per inch, each additional inch adds another R to the stack.
Second, in nature, the bees love trees, existed in them for eons before man intervened....right?....I am sure there are caves and underhangs and such, but in general.
Now our hives are made from "1 inch" thick wood that is really 3/4 inch, so R value of a very generous 1.
The dead air space inside the hive has a value, and again being generous, lets throw a full R for that, now up to R-2. On a CALM day there is another half R for the air around the exterior. Max R-value of 2.5.
Yet in a tree, depending on how huge it really is, can be surrounded easily by 8 inches of solid wood on all sides at a minimum. Lets be generous again, cut that number in half and say it is only 4 inches thick, an R-4. Adding in the 1.5 for the airspace we also gave to the hive we now have an R-5.5. Without showing all the math, the R-3 additional that the tree has is huge figuring a brood temp of 95 degrees....
I am surprised that virtually no one insulates hives....or there isn't much info that I found on it. Using a 1 inch polyeurothane foil-backed insulation (Thermax is a brand name) just by putting it on the side of a hive you add an additional R-8 to the 2.5, making now R-10.5. But if you can create an air space of 1/8 inch between the hive and the insulation board by using strips of ? 1/8 thick, you effectively double the insulations value. That hits R-18.5. In my mind that says to me that the bees don't have to work so hard to keep the brood warm, if your temps drop below freezing, it is a real concern I would think.
I do not think that really is much different than a good old thick tree.
Now I wonder why in nature the bees frequently have an high and a low entrance, exit? Some folks swear you gotta have a top only, or a bottom only exit, don't see much about using both. The bees in a tree can't close it off, and probably don't want to as it vents out stale air and unnecessary moisture.
So, it seems to me that much of what is "required' to do bees right, is quite opinionated. Me thinks the bees know better than we. But I bet they would like an insulated box in cold climates.
Please don't belittle or get too contentious.