I hate to voice the dissenting opinion but, anything is better than foil. Why not use mylar? If you don't want to buy a big roll, you can go to one of those party supply or store that sells gift wrapping paper they usually have small sheets of mylar and its not much more than foil. Or you can always use that white plastic sheeting many growers are now using although its a little pricey for my tastes.
I suppose that foil is better than nothing. Bottom line, if it works for you thats all that matters.