Rigid foam insulation is an expensive choice, and I have no idea if it will ever pay for itself or how long it will take.
A lot of the cost seems to be that it is a very uncommon method for residenti...
It seems like so much material, but the engineer did the calculations, and six inches of rigid foam, along with sheet metal roofing and solar cells (eventually) are actually less weight than traditional asphalt shingle roofing, so no structural changes were needed. And unlike shingle roofs, it should never wear out or need to be replaced.
1