Insulating Containers

I am just starting to plan a container based house to be situated in the mountains of Japan where temperatures drop to below -15C. It appears one of the big issues I will have to resolve is insulation. I am really hoping not to change the exterior of the containers if possible. Does anyone have any suggestions on products etc that I can review? Im up to speed with cold room insulation but looking for other alternatives. Thanks.
Thanks, I've looked into SuperTherm but am always vary of companies making amazing claims like they do. There are even a couple of articles on the net claiming it is all BS. Anyone got 1st hand experience with this product?
Do you have the containers already? If not, have you thought about sourcing reefer boxes instead of general purpose? The insulation in those can be really effective, is already installed, and you won't have to modify the outside in any way. You'll probably have to use hi-cube boxes in order to maximize internal height, but you'd likely have to do that with GP containers anyway - they're only about 2.35m interally, and that'll be reduced once you start adding insulation/ceilings/flooring/etc.