They're mylar, but it's very, very thinly deposited metal on the film. Since they're designed to reflect heat, they don't have to be so thick as classical growing mylar. If you're in a bind, and have the rest of your room all reflective, but say, you want to hang a shroud around part of the room, or maybe shroud a reservoir or something, they're ok, but they're not good light block/reflect objects.
I read up on the various incarnations of mylar a long time ago and that's the basic deal, as I remember. Any errors should be attributed to me not knowing wtf i'm talking about lol.