I've used a single layer of 70% aluminet (13X 21) for two years on the playa, and I think it works very, very well. It seems to drop the temp at least 10 -15 degrees from direct sun. I don't have data to prove it, so maybe it is less, but I think it makes a huge difference. I suppose two layers would be better, but I wonder how much better. It is also very strong, when tied right. We secured it by simply tying it with a Sheet bend knot, Cub scouts. Zero fly-away in even the worst storms last year.