Some folks were asking about different foam types, I’d say go with the polyiso, it’s a better insulator. The reason I went with the EPS is more to do with cost than anything else.
There is a rule of diminishing return on engineered or installed R value for a given structure, and for my needs, I felt my money was better spent on other items. For example, I plan on insulating the basement from the underside, and the doors for the basement with the same “cheap” foam, and also applying a bed liner cover over the insulation in those areas to keep up durability.
If I had gone with the polyiso, the funds for doing that would have gone to just the foam board itself. Additionally, R value is one component in many to make the insulation effective, and I think the returns on the R value difference will not be all together that much different.
Here’s a toy to play with some numbers:
http://chuck-wright.com/calculators/insulpb.html
There’s a huge body of work related to this. I am not a civil engineer, I just stayed in a holiday in express one time.
I also purchased stock in the Dow chemical company buy buying about 30 cans of “good stuff” spray foam (red can). With the nozzle I am spending time injecting spray foam into the crevice and gaps around each panel installed in the bus. Then, I’m shaving it down flush with a wire wheel.
After that, I will tape the seam with foil tape, and apply an elastic foam tape that’s about 1/8″ thick. Then, on top of THAT, I’ll be installing 1″ eps foam board (foil facing inwards) jointed to the wooden support boards. If I’m feeling jaunty I may spray foam the gaps between the wood boards and the 1″ boards, but most likely I’ll just do a close fit, then foil tape the 1″ boards to the wooden support boards you seen in the pictures. I fully expect the interior to look like a giant jiffy-pop pan when done.
On top of that, I’ll install the finish panels, which I’m trying to decide between white ABS plastic with a fine sand texture finish, or .060 FRP panel with the same texture finish. I’m leaning towards ABS due to it’s ease of install.