Perfect warm passages.
Again, the 2-3 pencils as stated above do the trick.The hard cover gives better support to foam insulation board when used for top insulation.
The total volume above the frames created by the hard cover is considerable and may amount to 1-2-? litres of air that effects wintering.micro-climates, those are created in the cells and at the comb's surface and would not extend for any distance.
Bees continuously loose energy to that air space - it adds up - they are, essentially, heating the attic.
The soft cover prevents that (and reduces the energy (i.e. honey) needs).
Typical honey needs for the winter in E. Euro - 20kilos (50 pounds).
Partially due to common use of soft inner covers.