When To Remove Bottom Insert iN Hive

Here in Northern Wisconsin, it's been a cold winter and I lost all my three hives. I installed the three replacement hives (and Queens) last Wednesday afternoon. Its been getting down to low 30's and below at night although yesterday during the day it was 65. I was wondering about the masonite insert which I placed in the bottom of my screened bottom board last winter when I closed up the hives for winter. I've left it (the masonite insert) in the screened bottom board so far to prevent the cold wind blowing into the hive at night ..especially since I've got the new bees (and queen) just installed and getting established..I'm leaving the entrance excluder open at the smallest opening until the hive gets stronger..perhaps in four or five weeks..weather permitting. Thoughts and comments would be appreciated.