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Apparently there is Extruded polystyrene shortage in NorCal and the clostest Home Depot carrying it is 200+ miles in the Bay Area. I was pretty set on either using Dow Styrofoam (blue) or Owens Corning Foamular (the pink panther kind), both XPS, in combination with dynamat and relfectix but the unavailability of it has made me reconsider Polyiso which is readily available. It's hard to know what the true pros and cons are as both product lobbys have publised their own studies regarding R Value retention and moisture resistence. XPS: -R Value more stable in lower temp -more resistent to moisture -more outgassing and way more greenhouse gasses -toxic fire retardent outgassing - lower melting point which may be reached behind dark sheet metal Polyiso: - higher base R value but drops by 25-30% around 30F - R value may degrade long term but probably less of an issue inside an RV where it's not expose to the elements as it would be outside a house -may absorb more moisture but issue reduced with foil facing. also the tests of submerging it in water seem extreme compared to the moisture it is exposed to in a camper so the technical difference of absorption may not be relevant in a camper -more fire retardent -less outgassing and no toxic fire retardent gasses -almost no greenhouse gasses during production -higher melting point of 260F -easier to attach and seal gaps between panels I think in the end both are probably fine options and not wanting to drive 8h round trip to get 4 2x8 panels of XPS I may go with Polyiso. Polyiso seems to be used more frequently in foam board insulation conversions, not sure if that is because it's better or because no one can get a hold of XPS. Thoughts or experiences anyone?