Hexacomb is a possibility. It's just not as easy to find as polyiso. And I don't know that it has a foil outer layer. I'm guessing it doesn't, which means it's not ideal in case it rains. To be prepared for rain, you'd probably have to tape the outside with foil tape I imagine.
As per the Hexayurt Playa webpage (
http://www.appropedia.org/Hexayurt_playa), hexacomb apparently also needs an additional insulating layer, as in and of itself, it seems to be insufficient for insulation purposes.
R+Heatshield is specifically mentioned to be used in conjunction with hexacomb.
Also mentioned are Coroplast or Triplewall corrugated cardboard. But again, I'm guessing they too will not withstand rain on their own.
Personally, I think 1" polyiso is the way to go. I built an H13 with Rmax. Light, sufficiently strong, and great R factor to weight ratio, plus it already comes with an outer foil layer (which reflects sunlight away as well as resisting rain).
-Alejandro
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-Alejandro Moreno S.
GreenMBA, Dominican University of California