Just a couple of observations:
When I kept bees in France, it was the practice to put one or two  2
inch screened holes in the bottom boards for ventilation in summer and
winter.  When I moved to southern states, I did the same  and  found
that the bees propolized those shut.  Recently, I visited Morten P. in
Denmark (cold climate) who uses bottom boards which is nearly fully
screened.  While the the majority of the hives are made of a dense
insulated material, the bees did not propolize the bottom screens.
Ed