I posted some of the info directly applicable to this a while back on a Sprinter forum (naturally, it fairly quickly turned into the usual p*ssing contest.),
Anyone who is interested might start around here:
Of particular interest to those who overnight in very cold temps is the chart in this post:
The cliff note version is:
1. radiant barriers only work if there is an air gap,
2. the best place for a radiant barrier is outside the conditioned space, but inside is better than none at all
3. adding a thin foam or bubble layer in between two radiant barriers (LowE or Reflectix) is not as good as a true air gap of similar thickness, and
4. thin foam or bubble layer adds practically no R-value to a radiant barrier,
5. Polyiso spray foam performs poorly in cold temps, but that is *not* the case for all forms of PU foam, in particular, Dow Thermax, which outperforms XPS (e.g.: Foamular) by 30% even in cold temps.
6. The exact insulation scheme depends on whether the design is biased toward limiting heat gain (hot climate) or limiting heat loss (cold climate).
7. No matter what the climate, Thermax (or Tuff-R) is just about unbeatable for thermal insulation, at a fraction of the price of Thinsulate
8. Materials designed mainly as acoustic insulation (e.g.: Thinsulate 600L or hydrophobic melamine foam) have only about half the R-value (relative to thickness) as Thermax, i.e.: similar to ordinary fiberglass "batt" thermal insulation.
9. Foams designed mainly as thermal insulation (e.g.: Thermax or Foamular) aren't nearly as good for acoustic absorption as most fibrous insulation. But acoustic absorbers aren't very effective on low frequency noises (e.g.: a diesel truck idling nearby), which requires a limp mass barrier (e.g.: mass loaded vinyl).
10. Thermax is also fairly rigid, so you can't stuff it into tight cavities. But that also means it can serve as a reasonably supportive substrate for whatever veneer is used for the finished surface.
And yes, Thermax does comply with FMVSS 302
, Foamular (XPS) does not.
Here's a few links to some of the studies / tech info that have guided my real-world experience:
(chart on page 62)
(see chart on page 2 adds Thermax to the chart in the study above)
Here's Owens Corning's attempt to muddy the water by conflating Polyiso spray foam with PU foam sheating:
(chart on page 2)