For the 3.476³ time, closed-cell foam is not perfectly closed cell, 97 or 98% is the best they can do. Since Airstreams never leak and condensation is a non-issue maybe you should stop reading and jump to the smileys after this next text block...
The comparative foam volume of my trailer main shell at 16' by 18' with 1-1/2" thickness of foam would be 36 cubic feet. With 3% open cells that is one cubic foot, or 62 pounds, or 7-1/2 gallons of water storage possible.
If we count only the nearest tenth of foam thickness that is still 3/4 gallon of water holding capacity against the aluminum shell and spars. And that is only the beginning with unaged and unstressed foam, the cells continue to fail and moisture will capillary wick along random pathways throughout the total volume.
What we don't know about Avions can hurt us. The ALCOA OEM style protective plastic film that is so much fun to peel off new sheet metal would be a good start on having a spray foam protective barrier to keep water from undercutting and boring into the foam from any small defect, rivet or seam etc... IF the foam would chemically & physically bond and key into the surface.
I've seen water get underneath the factory Vulkem caulk schlabbered over the end cap interior seams to undercut its bond and run 16" from the leak and form a bladder holding water that squirted out like a clown flower when I was levering in the rear bath replacement floor. As gnarly as Vulkem is, time and freezing defeated it in a text-book worthy example.
I've also seen an old-guy in near-tears from sub-floor spray in foam that held enough moisture that the iron disappeared, leaving only an orange oxide stain. Looked like something you see on the banks of a polluted river, chunks of detritus randomized as he explored the damage once the plywood came out.
There had ought to be a law if you must foam it then you're obligated to keep the trailer for life, and store it carefully and do the maintenance.
A clean aluminum surface pretty much radiates zero infrared as a result of size and arrangement of its molecular surface. The @#*%^ spray adhesive used to adhere the OEM fiberglass in the shell hollows pretty much defeats that, and I don't know a good way to get the surface back to bare metal, can't get the 'clean' back once it's gone. Pressure washed degreasing and acid etched then passivation with an aircraft quality chromate treatment would be a good start and its not as difficult as it sounds, they are wipe on and rinse off treatments and a very good start to a forever trailer for countering the hidden weaknesses.
The question on a near perfect insulation is one that I've pondered more than a few hours on without fixating on the first shiny solution that appears... Right now I'm thinking its continuous Prodex (0.2") with a stand off from the shell by two layers (0.4") that is religiously tailored as a snug fit and perfectly foil taped to the ribs and C-channel... and then [drumroll please]
anything you want to use, even (gasp)
spray foam. The air gap and Prodex moisture barrier will allow liquid water to flow to the C-Channel if/when it occurs.
But that is not one I will use, just solving the spray in dilemma. I'll go with Prodex and pink housing foam bifold sheathing panels with a rib-to-liner thermal barrier of synthetic subroofing paper (not rubberized, just resin paper style)
Out of time, been away six weeks and a lot of catch up to accomplish...