Hi All,
I set up two of my hives here in Auckland, New Zealand using the ‘hollow
tree’ model and trialled it last year. The West and North hives were made
'floorless’ (240mm height above ground level) The 'sub-floor space' has an
enclosed base (or stand) but there is no screen or floor above the 'soil
floor'.
I measured the depth of the ‘bee cluster’ below the bottom bar of the brood
box at dusk.
Date           8April    15April   22April    29April   12May
North hive     40mm      55mm      68         40        37
West hive      30mm      55mm      50         55        45

I also used a digital thermometer to measure the temperature up through my
West hive. From memory the cluster depth of this hive was only about 20mm at
the time of the temperature scan.
The scan started under the brood box (cluster) and the sensor was lifted up
through the middle of the hive right up to the 'ceiling'. (the outside
temperature was about 20 degrees C on the day)


Position of sensor             Temperature/degrees C

90mm below bottom bar          19.3
level with bottom bar          24.8
100mm up into the box          34.1
200mm up                       34
300mm up                       33.3
400mm up                       33.1
500mm at top of hive           33.2
(It would be good to have some more data near the -50mm and +50mm levels.
It's not hard to do!)

I suppose the point I am making is that the bees keep the temperature
uniform around the brood.
Maybe the drone cells at the bottom of the frame in the bottom brood box are
a bit cooler.
If the screen is positioned only 20mm below the bottom frame bar and the
sub-floor space is open to the breezes, then the bees may not be able to
reach the desired brood raising temperature within the given 25mm, say. My
latest 'mesh screens' are positioned 50 mm below the bottom frame bar. (Some
drone comb is built in this 50mm space during 'spring build')

Cheers Paul b.  -- a cool day here in Auckland NZ.