i think foil is around 75% but same goes dont quote me either, remember tho reflector hoods for grow lights are made out of....Aluminum so it does have its uses, and yes i agree white paint is better and foil IS NOT ideal. BUT.... if you have no money and only a roll of foil then i would say use it up, and also if you already have lined with foil no sense in buying paint in my honest opinion. but anyways i dont want to debate over this just saying that it still is usefull. and i have a peeve about people telling people to take it down ASAP and go out and buy something else (not saying you said this but i have seen it numerous times on here) as foil is almost just as good as white paint, the only downfall is its more expensive and slightly less reflective.