Mine is a national hive from th----s , single standard brood will take rabs advice and partially remove while its so cold then take out completly when warms up a bit
Just to clarify- when RAB talks about top ventilation, he's referring to either something (eg matchsticks) propping up the crownboard to deliberately create ventilation, or the feed hole left open, rather than something built-in to the hive. Both of these have been traditionally done with solid floors, to prevent condensation.
However if you have ventilation top and bottom, this creates too much through draught. The current consensus is open OMF and no ventilation at the top, with the option of insulation on top of the crownboard.