Iso, at least in my research
into the issue when I totally started over (new ply walls, everything) on my box, is a crap shoot due to water
Even with the blue/pink (Dow/the-other-folks) extruded board will eventually degas and lose insulation value.
We used it instead, and, belt-and-suspenders, painted each block with epoxy
before insertion, to try to keep any possible water intrusion to a minimum.
I also used aluminum
foil on the entire perimeter, then door-skin battens for an air barrier, before laddering (making the joints stairstep) all my corners to minimize any possible heat migration in the joints. Once the box was built, and before I installed the top portion, I fiberglassed the interior
for strength and further air/water intrusion protection.
So, I had a radiant, conductive and thermal barrier to heat in my box. It seems to have worked well; when I decommission every time I have a shore excursion, leaving the boat on a ball, even with the doors open, it takes days for the interior
temps to reach ambient. Of course, the downside to that is that recommissioning similarly takes a long time to get the foam fully cold (I keep my reefer at 32/34, and my freezer
plate is set for 1.5/8 - which yields a 2° box hysteresis as well).
So, to your conundrum. Extruded poly, encased in epoxy
as you lay it, with air and reflective barriers as you go to the perimeters, would be my recommendation.
FWIW, you may be aware that even vac panels
will eventually lose their vac; I wish there was a way to easily replace my insulation, as I expect to own the boat long enough (it's already been over 10 years) for that to be an issue; I'll have to start over from scratch.
For any masochist who'd like to see that process, interspersed with the several other simultaneous projects, you can see it here:
Pictures: Flying Pig Early Refit + Projects/Early_Major_Alterations_Work
Start with the 12-04 and move onward...