I am finally back from a long work trip, I have been following this thread
but have been too busy to respond.

Under the right conditions (balance between insulation, ventilation,
thermal mass and volume) bees will indeed be able to maintain a comfortable
internal hive environment. If the loose cluster is generating say 8-10W
continuous waste heat via a mix of resting, walking, agitating and heating
levels of metabolism and this is more than what is being lost to the
environment, the internal temperature will be steady.

My thoughts is that it is all about the rate of cooling of the box (bee
nest) which is a function of thermal properties of the box, the amount and
type of ventilation, effective heat storage (honey) which is also impacted
by type and location of vents and the size of the box (space to heat and
surface area for heat transfer) . If the rate of cooling is above a certain
threshold, the bees will cluster to conserve energy. They will need to
generate extra heat in the cluster to keep themselves warm enough to
survive. The theory that the heat is conducted through the bees to the
outer bees does make sense. There is likely a combination of conduction,
radiation and convective heat transfer going on between the bees. The
colder the internal hive temperatures and the tighter the cluster, the more
the heat transfer between the bees will be via conduction (contact
transfer). The video linked below shows the bottom of a cluster where the
bees are sticking their butts out. I started the video showing bees in a
single hive. The 1st part shows (need to start at beginning) a double where
the bees form an hemisphere type cluster (I have data showing the above
temperatures during IR phototage) where the center of the cluster is nearly
at the bottom board (where IR camera was inserted and pointed upwards) and
the sides seams where the bottom of the cluster is progressively higher in
the box. Much more heat loss in a double brood and than a single brood
colony. It will also therefore be more difficult to generate natural
convective currents.

I also think there are other trigger mechanisms of clustering behaviour. As
I have seen many times over the last few years, the bees will trigger
heating events that will last for 2-3 days before temperatures revert back
to an equilibrium (the higher the R-Value to slower the heat transfer out,
the longer it takes for heat to flow out, the more honey mass the more the
heat is stored). Most have been associated with extreme cold events and all
created condensation events where the water was 15-20C. Remember, at -40C
bees must generate 100% of their water needs to not die of dehydration.  At
-5C RH 80% bees need 3 to 4 times more moisture than is available in the
outside air. I currently have about 40 days of CO2 (Ti (10 to 16C), To (4
to -33C) and RH%) just above the cluster.  You can clearly see a series of
CO2 cycles. I also see some interesting CO2 spikes above 1% mostly linked
with dropping temperatures. Unfortunately I don't have an array of sensors
above this cluster to understand its behaviour (day job is too busy) but I
will be able to associate it with previous observed behaviours.

Another possible driver would be to create what I call vent channels. This
is where a portion of the cluster is loose and they create an exclusion
area which coincides with the outside seam. My thought is, it is to create
a clear convective vent channel where upper temperatures are in the low 20s
(C) and the "used" air is allowed to cool a drop out.

It has also been shown that insulation, will delay the clustering behaviour
as the temperature drops, I will also postulate that (modified
Southwick/Seeley Metabolic rate chart attached) the reason for the
metabolic hump peak around 15-20C is due to enclosure heat loss and the
bees attempting to maintain a higher brood nest temperature (35C) without
the benefit of the cluster. The delta T is still 10-15C times the R-Value
and the surface area in heat loss (Q = U * A * ΔT)

Note: in milder climates beekeepers can get away with no insulation. I am
happy with my setup where bees consume less than 30-40lbs of honey over 7
months. The challenge is to find the right balance hence the reason I enjoy
these discussions and theory deep dives.

As shown in one of my videos it shows a mix of double brood and single
brood colonies.
https://youtu.be/YbSgHJMgxuY?si=tsm9GuhPtbP3pf66&t=204

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