From what i have read mate Its not a good idea, in fact its a real bad idea because the foil causes hot spots. Get some black on white PVC from a grow shop. Its real cheap and great as a reflective surface. Or if you have time and im expecting you do if your going to foil your wall's. Is to paint the walls white with a paint that wont go moldy when wet.
I think you mean aluminum foil, tin foil I imagine would be hard to purchase. Both are ineffective for reflectors, use flat white paint, panda film or mylar. Htgsupply.com has decent prices on reflective materials.