Be sure to check how warm it gets after it gets dirty or dusty. It will probably reflect 90-99% of all light and heat while clean, but who know how much it will absorb when dirty?
This is only from reading, not experience, but I've read that flat white paint reflects lots more light than standard aluminum foil.
But I think that designed-for-the-purpose metalized film is a better reflector than kitchen aluminium foil. Is it better than white-painted surfaces? Googling shows different opinions.
Gloss white paint is easiest to keep clean, and dust can cut reflectivity more than the difference between different kinds of white paint and aluminized Mylar.
