What if you put something between your hand and the stove?
It seems that you are rejecting the idea of "insulation" in general as a result of having specific questions about R-value.
The basic reason that a big balloon pad isn't the warmest possible design is that the air in the pad gets cold, and you are very close to that air, despite being several inches from the ground. It's easy for the cold air to rapidly get to you by flowing around in the pad (convection). In a piece of foam (even a thin one) there are many air spaces between you and the ground, separated by many little plastic walls. The cold air near the ground can't get to you directly.
This is the reason that a given thickness of foam is a better insulator than the same thickness of air. R-value is just one way that people measure that difference in insulation. If you want to know how R-values are measured, you can google it or read the article that is currently on BPL about Roger Caffin's method.
