I wouldn't worry about insulating them, but I would get rid of the sugar block and feed them 1:1 warm syrup instead. It is not easy for them to consume the sugar block.
I would suggest putting the inner cover over the bottom box with bees and then inverting a jar of warm syrup over the inner cover hole. then the empty 2nd hive box and top cover. Make two jars of syrup and keep one in the house, every 12 hours sway out the jar on the hive for a new one.
The bees will keep themselves warm if you provide them with food that is easily consumed (warm syrup, they will not take cold syrup) they have no brood yet that they need to keep warm. Since you are in NC, the low temps will only be temporary and sporadic. Syrup will encourage them to draw wax, sugar alone will not as it requires them effort to process. I have used this many times here in NY in March with newbies and it works great. Just keep them with syrup they can consume 24/7. Since you are using mediums you will need to keep any eye on their growth and add the second box quickly. I always use deeps for brood boxes.