Plain white. Tin foil is one of the worst reflective surfaces because it crumples under heat and reflects light at oblique angels, yotta yotta yotta. don't use it. Search the forums be for you post.
Hi mate, If you are stuck for cash, pick up some cheap emergency blankets. They will do exactally the same job as mylar, but for less! Plus the paint fumes can sometimes be an issue. Hope this helps. mallett
I went to Home Depot and checked out Mylar and that stuff is expensive. Guess I'll just stick to the white paint. Thanks everyone for the replies.
Go to an army surplus store, a sporting goods store or a camping store and you can get emergency blankets (cheap mylar) for around $2 to $4 a pop!