Quote Originally Posted by hoodedclum
Yeah tin foil actually absorbs more light than it reflects, your better of with a bare wall.
That is incorrect. Aluminum foil has about a 90% reflectivity (Aluminum is an excellent visible light reflector, actually. It is used to coat telescope mirrors, and with enhanced coatings can achieve >98% reflectivity). The problem is not the reflectivity, it is the lack of diffusion. The shiny side of the foil creates hot spots and dark patches, because of the smoothness of the surface - it doesn't diffuse the light, the way white walls do. The dull side is a little better (depending on brand - if you cannot see a clear reflection of yourself in it, it's suitable), and will do if you have absolutely nothing else available. But, Mylar is better in the long run, and it is also more durable. Foil tears very easily.

Flat white paint is the very best, as Zandor says, since it completely avoids hotspots and it can have as high as a 92% reflectivity, give or take.