I need to know what is the lightest air-tight material or combination of materials?

I would really appreciate some help with this but if you can't at least tell me where i might find the answer

Have sort of been thinking of suggesting Mylar or aluminized Mylar. It is used in both party balloons and in aerospace applications. Fosset may have used it in his recent around the world trip. It was very important in some of the early assents high in the atmosphere.