Mylar or Flat White Paint?

I used a walmart emergency blanket, which is the same thing as mylar. It was $2.59 + tax or something close to it. It's really inexpensive. I originally painted my grow box flat white, but then decided to try mylar. Afterwards I immediately noticed it was much brighter in my 2 x 3 grow box. However, flat white paint or mylar will do the trick just fine. I'd recommend mylar over flat white paint though. Just use thumb tacks to stick it up and, of course, make sure you lay it flat against the walls. As far as heat goes, it's tin foil that you want to avoid because it creates heat spots. To the best of my knowledge mylar doesn't create heat spots. If by any chance you're using fluorescent lights, you don't have to worry about heat anyways.