On my bus, I used a spray/brush on ceramic insulation/coating (I think the new commercial name is Lizard Skin) on the inside of the exterior skin.
I then added a layer of the "bubble wrap" type insulation with the foil on both sides. Not the best insulating properties, but it shouldn't be an issue when it comes in contact with moisture from the design of the side windows.
I am also in the process of adding a 1/4" layer of closed cell landau foam on the interior side of the interior skin. It will serve as a padding for the upholstery as well as have some insulating and noise dampening qualities.
