I just did some experimenting with some space blanket material on my skylight as a reflective material to help keep my house cooler.
I thought it was a great idea, but the space blanket material ended up "melting" to the glass and turning clear in most spots!
I used another piece on my downstairs window, and that works great! What's the difference? I think the skylight gets A LOT more light than the window. I don't know if the angle of the glass had anything to do with it.
The other thing I would check on is the flammability of the space blanket material. Take a small piece, and hold a cigarette lighter to it. See what happens. Now imagine an entire attic like that.