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It's actually mylar that they're made of.

https://en.wikipedia.org/wiki/BoPET

I believe besides acting as a wind barrier, the foil reflects heat internally. They would not be effective in direct contact with skin and another surface.

I spent a night in the woods with a Mylar foil blanket. It blocks wind and reflects the heat back to you. When it came loose I would immediately wake up cold and reposition the blanket. The thermal effect was instant on and off.