All self-inflating mats I have seen are self-inflating in the sense that you’re supposed to inflate them yourself. [That is, the mat self-inflates a bit, but mostly you have to breathe the air in.]
Given this, what purpose does the foam inside the mat serve? If there was none, wouldn’t the mat be lighter?
My hypothesis is that the foam prevents the air from moving inside the mat, thus greatly increasing the heat resistance. Is that the case? (Do you have some dependable sources?)