The problem with the foil/bubble wrap insulation is that it is designed as building insulation not as a sleeping mat. It is not really rugged enough to go between you and the ground. You will end up punching holes throudf the foil and popping bubbles. But it does work great as an add on bottom layer for a hammock. But as someone else said it is a bit bulky.

By the way I know about both cases becauuse I had the same idea a while ago and tried it. Ended up with a Swiss cheese mat. Which is not that much padding to start with. Not that comfortable at all. As lower insulation for the bottom of a hammock it works especially in winter camping. But not any better than a space blanket. The spase blanket contributes much less bulk to the sleeping system.