Don't use tin foil it can create hot spots and burn your plants, Use Flat white paint that's cheap or free you alway's know some one with some extra paint.,,,If you still use the foil use the shiney side down,,,just my 2 cent's.
flat white paint would probally be the cheapest then there is
Mylar and Panda film which does get expensive
you could probally paint the room for 15-20 bucks