Is it really necessary to insulate a National Hive?
Things have moved on since the 1920s when the hive was specified to pull together a standard from the proliferation of sizes at the time.
We now have OMFs with no need no top ventilation.
Years ago 'quilts' were used as cover and insulation, not simple coverboards. Some used sawdust in a box above the cluster/brood, some used straw in a sack, others doubtlessly used other materials and different implementations.
150mm roofs are much better with respect to adding insulation above. Just that most roofs are bought/supplied, these days, on a 'cost' basis rather than a 'thinking ahead' basis.
Bees to replace winter losses were cheap and easy to obtain, many years ago (but not including the years of the Isle of wight disease!). They are now a deal more expensive so relatively small outlays on insulation, to safeguard what, to some, is a sizeable expenditure is not a bad idea, I would have thought.
We are all encouraged to insulate our homes - think back to the early 1900s and compare building practices to those of modern homes. Some comparisons here for the homes of the bees as well, I think.
WBCs are generally thought of as more cumbersome, less easily moved, slower for inspection manipulations, etc. But they were, and still are, a good hive from the point of view of over-wintering. It is just that modern agricultural practices have changed and beekeeping practices must change as well.
It may be a flaw in many designs/formats, not just Nationals. Hives are used from almost equitorial climates to harsh arctic-like winter climates. Insulation requirements for these different areas is never going to be the same! On the same theme, but with less extremes, the National is really a UK (English, even?) design standard, but there are great differences in the harshness of the winters from the far north to the far south and south west regions.
The U values of modern insulation boards are far different from the carpet squares used in years gone by. There are other things to consider here too. Disease transmission, for instance.
So, a little thought and a list of pros and cons might have answered the question.
Hope this encourages all to make a list before posting. The question is good, from a point of making some non-thinkers aware of the alternatives of then and now, but is easily answered simply or more substantially.
The above points are not a written 'brainstorming' list followed by expansion of each possible line of thought , but just of a few ot the factors that came to me as I was writing this reponse. I would think there would be several more additions had I thought more deeply on the subject/topic.
Regards, RAB