It helps alot to attach (velcro or other method) reflectix to the outside back wall and sides of each cage if you can. Will help with your heating costs. Works best if it is inside the cage, but that is not always an option.

Not sure about the moisture. You could lay down some polyethylene sheeting (sold for use under wood floors), lay pieces of MDF down over that, then the styrene foam, then the cage and test it that way. Check periodically to see if the MDF is getting wet. Not sure why it would, but I don't live in a cold climate.