This thread makes a lot of assumptions.  

Whether bees cluster as expected at the expected temperatures depends on type of bee, state of health, including parasites, history of the ambient temperature and humidity, presence of food and water, season, amount of brood and many other factors, amny of which are unknowable.

Only one thing is certain. There is no ideal hive and no ideal hive configuation.  Beyond that, the answer(s) to the questions asked here are as varied as the beekeepers asking them, as are the assumptions and unknowns. 

Bill reminds us occasionally that all beekeeping is local.  I would go as far as say, as local as your yard and as local as your own mind.

So much depends on the region, location, the operation, the goals, the bees, the mangement history, and many other factors that finding a solution is a logical tangle with many parameters, some in opposition and some augmenting others.

Generally speaking, generalities are at least partly wrong.  At best generalites  only describe the imaginary median case under median conditions, "All other things being equal".  Well, they never are.

People demand answers and it is too easy to get some empirical or extrapolated  data, make a scatter chart, draw a line up the middle and base decisions on that line when all that line can actually tell you if you are being honest with yourself is that realites lie at varying distances on both sides.

Sometimes data points are far from that line and the tendancy is to discount or discard them when, fact is, they are the most interesting.   Each case, each dot, is unique and has its own story.

In my experience, wintering for decades and inspecting wintering hives in Alberta and elsewhere, insulation helps, some years more than others, and for weak colonies far more than strong and healthy ones.

Placement and amount of that material depends on the other aforementioned factors.  One very successful Alberta beekeeper insuated the top, back, and sides but had only black plastic on the south facing fronts with an auger hole for safety entrance and ventilation.
  
He was balancing the need for protection from wind and heat loss on the sides away from sun with the need to take advantage of solar gain when available.  The reasons for that are a separate topic.

FWIW, this has all been covered in detail in the archives with associated topis as well.

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