I have been using screen bottom boards since I started with the bees last year. This year I have two nucs going right now and I have sealed off the entrance down to a very small opening, similar size to the entrance reducer on the smallest setting. I have done so because the colonies are small and the temperature at night is still getting down into the 30s Fahrenheit, tonight is forecast to be 29 degrees F.
However, the nuc boxes I have are solid bottom boards and I was wondering yesterday evening whether I've restricted the fresh air flow too much.
--John
I made my nucs just like the standard hive. I have inner covers with a screened 2.5" hole in the center. I just raise the back of the telescoping top a little to let the air flow through. If I expect the night to be cold, I just lower the top cover. That closes the inner cover hole! :-D
I have polystyrene nucs with just a 3/4" entrance hole and they do great.