agreed, mylar is good for eliminating hotspots but it still concentrates light, flat white paint disperses light evenly over a larger area, much much better than tin foil, using tin foil is like holding a magnifying glass over your plant and concentrating the beam of light into a dot... just all over the place.95% of the people on here will tell you not to use aluminum foil, for good reason. There is, however, an exception. The ONLY way I'd recommend using foil is if youre on an EXTREMELY tight budget, and are using CFL's for light. DO NOT USE FOIL WITH HID's! CFL's are not powerful enough to cause the hot-spots and burns on your plant that you will get when using foil with an HPS or Metal Halide.
A much better solution is flat white paint, or if you have some coin, Mylar.
As I recall, flat white paint is around 80 (something) % reflective and aluminum foil is only around 40 (something)%. That's the real reason not to use it, it just sucks!- - - - White paint is a little less reflective, but it's a very good alternative.
I don't buy into the "Hotspot" story! I've never experienced it myself and I don't know anybody that ever has. Or even anybody that claims to have actually seen it!Out of curiosity I always wondered why tin foil is different than mylar. They have about the same reflectiveness. Why does foil concentrate light more than mylar? Not that I disagree, just opening a little debate and discussion.