Have you looked at extruded polystyrene as a core material?

In all the material lists I can find, the harryproa's list h-80 as the foam core material. While its performance is great, its costs are similar.

Extruded polystyrene is a closed cell material available in many grades with increasing performance with cost. The highest grade is about half the performance of H-80. The plane guys make aircraft out of it.

Perhaps this is nibbling at the edges, as it seems the harryproas have dropped the cost of building so much.

Thanks Rob.