He was asking if people just keep shouting the same age old theory that mylar is way better than foil for reflective purposes, or is there truth in the statement.

Also. the "example" you asked for in the bottom of your post about heat reflectivity---think infrared detection equipment. They woould not be trying to "keep heat" inside the room, just keep it from being visible to IR detection.
ProGroWannabe Reviewed by ProGroWannabe on . Mylar vs. foil For a long time I assumed everyone knew what they were talking about when they said not to use aluminum foil, and to use mylar. Apparently foil can create hot spots. But here's the thing, so can mylar. In fact, it's often used precisely because of its ability to reflect heat! Anyway, I've heard all sorts of misinformation, like that aluminum foil is only 40% reflective whereas mylar is 98% reflective. So I looked up some data in my thermodynamics textbook from college. Actually, Rating: 5