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.
Too slow for those who Wait,
Too swift for those who Fear,
Too long for those who Grieve,
Too short for those who Rejoice,
But for those who Love,
Time is not.
-Henry van Dyke
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.