with the really good ones there isnt any space, just insulation. Camping Gaz used to make the finest.....you could take two 45 litre boxes with blocks of ice in and they would last a week if you started with one and kept the other closed.
Coleman, who own CH, are not bad but not quite as good. Cheap ones use just the gap but you will only get a short term effect.
With high quality ones, you can even use them as hot boxes. In Egypt, Oman and many ME countries, CG boxes were used by coffee shops, and also attached to hawkers bikes to sell hot coffee on the streets. They just work the other way around, like using double glazing in hot climes to keep the cold in and the heat out.
I used to work for CG.....en France