I kept an empty box (just foundation) on the bottom of a weak hive for the last 6 months (from late fall to early spring), my thought was it would act as an additional wind break and let them regulate the temp better. I think it worked out well.
What I saw was they totally ignore the empty box, and just stayed concentrated in the second box, to the point of 100% utilization.
Since it was totally devoid of brood and stores, I now have put it up on top, within just a week it was full of brood, where they previously just ignored it.
Given your location, for the time being I would just leave it alone, until it warms up. Then if they still ignore it, then put it on the top of the brood chamber.
btw, I will probably do the same thing next year. I really like how it worked out. So next year I will put an box on the bottom, but just frames, no foundation.
I had no condensation problems, and the hive build up fast in the spring, much earlier than I expected.