The reflective insulation they use in house attics really needs air space to work properly. In other words, nothing should be touching the reflective side, as little as possible anyway. I gather the reflective insulation in an air mattress works the same way, so if it's stuffed with down, it may not be of as much benefit.
If anyone has built a large tarp with a catenary curve before, I'd love to hear your input on how much droop (the amount of drop in the center) to give it. Getting everything lined up and taped properly is really tedious with survival blankets because there can't be any wrinkles to let in water, so I'd prefer to only cut and tape it once rather than try a small curve and see if it needs more (wash, rinse, repeat).