I agree with Robo. Get some syrup on them. Over the vent hole in the inner cover, they can work it pretty hard. I wrapped mine in roofing felt too, and they seemed much more active on marginal days.
Since this is my first year with bees in the snow, I was worried about them overwintering. I had one colony that I was very worried about, very lite. Then we began having a kind of false spring the last 10 days. Bees were flying like mad, so I popped them open for a look. I think there are more bees now than there were when they stopped flying. 2 full boxes of bees and pounding in the pollen like a UPS feeder line. The bees are alot darker than when they quit flying in the fall. I don't know if I blinked and had a supercedure before the weather turned nasty, but I like what I'm seeing today. If they make any honey, we're gonna have some queen rearing material.