Here is what a shipping container house builder in Texas told me about how to prepare a shipping container for exterior insulation and cladding:"Weld some steel strips to the exterior in a box form on each side you will be cladding. Make the down posts some smaller tubular steel-1 inch perhaps, then use just flat strip steel about 4 inches or so about a 2 ft from the top and 2 ft from the bottom, and a third halfway between those. To that screw on some good flat 3/8 plyboard. This will give you a good pocket for the insulation to go in, and a great base to attach your cladding to without puncturing your container any more than necessary- a definite best thing to not do! It also creates a better situation if you have a storm that rips any cladding off, as it doesn't rip through your container!"
The suggested insulation is closed cell foam. I am wondering if boards of Eastern Red Cedar (Juniperus virginiana) would work as the 'plywood' and cladding? Of course ERC is highly flammable in tree form- it's explosive, but what about boards? And would smaller diameter boards be more flammable that larger diameter? If I butt them tightly to keep fire from getting at their edges will I have expansion and contraction problems? How long would I need to dry the boards after milling before using?
If the ERC is too complicated to use, what are some less expensive cladding options? Something nonflammable.
Your description sounds a bit like what I did on a short wall.
The wall is 3 feet high, so using 6' cedar fence boards was a very efficient use of materials.
Cedar reacts with most metals, bleeding out dark stain. So part of my method was meant to reduce that issue.
I screwed pine header and footer boards and filled the space in between with solid foam board. I pre-drilled holes in the cedar and used hot dipped spiral siding nails to attach at top and bottom.
Choosing the boards, I was very particular as to straight edges and heartwood color. I stain-treated all surfaces of the siding boards before installation. The pieces butt up against each other so well that there is barely any space in between. If a board showed any bowing, it was discarded.
I thought it was a very cost effective way to get cedar siding.