If I insulate a surface with two sheets of rigid foam insulation with thickness x, or with one sheet of otherwise identical rigid foam insulation with thickness 2x, how should I think about the performance differences between those two situations? What about if there was an intentional air gap between the two?
I'm thinking of an application exposed to cold air, around 10 to 50 deg F (-12 to 10 deg C). I'm also assuming that the warm side of the insulation would be faced with aluminum foil. I'm especially interested in expanded polystyrene.