< The bulk of the cluster lives against the insulation and the remaining part is in the beeways directly below.
The thermal material properties of that top insulation (~>2" styro) is better than anything the cluster can muster. In my setups I have seen the bees use sidewalls (doubles) as their preferred location. I stopped using large candy boards as the bees never really got into them. I do have a small feeder slot built in to my top that I use do add fondant, medicated syrup or spring patties.
The main reason for me focusing on this aspect of beekeeping is we (In Canada) lose over 200K hives every year (winter losses ~30-40%). 25% of overwintered colonies are indoors. Indoor wintering have higher losses than those outside (33.4% Vs 28.9%). Main reasons stated are weather, starvation, weak colonies, queens. (https://capabees.com/shared/CAPA-Statement-on-Colony-Losses-FV.pdf)
We still have a long way to go to find the optimum approach for the various regions. I believe the impact of varroa on the colony's ability to effectively thermoregulate (make decisions) is understated (not well understood). My carni mutts do best. My yearly survey (150 to 200 hives) clearly shows carnis do better in cold climates even those that I get from California/Southern BC.
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