mylar is the best reflective surface but is a bitch to install and even more of a bitch to keep clean. it also can cause mold to grow between it and the wall from lack of circulating air. and unless it's kept clean it's no better than flat white painted walls, which are next best and most often used by growers for its easy application and ease with cleaning. aluminum foil might look good to newer growers but is actually a waste of foil and time as it soaks up needed spectrum and creates hot-spots. mirrors are the absolute worst to use...the reflect very little beyond 'white light'.

just stick with 'the book' and you'll have a harvest worthy of your time and effort.