In uk.d-i-y, David wrote:

the pot is in a hole in the insualtion but the top is not covered with
insulation.

Then you'll get radiation losses from the top of the pot, but not much
in the way of convection, since there's not much chance of air getting
to the bottom of the pot to start an updraught. An old lath-n-plaster
ceiling will provide more in the way of airgaps for convection than a
nice new plasterboarded one - to get convection going there you'd want
to stand the pot on some spacers, or nibble some gaps into the rim (as
if the little bleeders won't crack if you try ;-)

Stefek