Hello,
I am still in the design phase of a cargo camper. I have read the insulation topics and would like follow up with some questions for the experts!
I am assuming that AC and heating will be necessary, but would like to have enough insulation to quickly cool down on the hottest days, and get it warm on the coldest.
Due to the thermal conduction of the metal skin and ribs, it seems that placing insulation of any type inside the wall is not going to be that effective. The 3/8 plywood interior does provide a thermal break although not that much R value. It seems that gluing a 1/2" layer of foam board over the plywood would be much better than putting it inside the wall. (The box within a box idea). A thin layer of wood paneling (or whatever) would then be glued to the foam board. Of course this costs 1" of interior space which could be a problem for some.
If the insulation in the void is really helpful, then I like the idea of spray foam instead of cutting sheets. There are companies that do this for homes by drilling holes in the top of each wall void. The foam is then squirted down inside the wall. See retrofoam.com for a video of what I am talking about. Obviously the expanding pressure is low because a product like Great Stuff might expand sheetrock, and certainly puff out aluminum skin. One big advantage of doing this would be extra rigidity to the skin against bumps, etc. Compare to the cost of going to a .040 or .050 skin.
As far as the ceiling, it seems cutting the foam board and placing in between the ribs would not be as effective as placing it completely under them and leaving a void between them. Then attach furring strips for mounting luan.