homeconstructionimprovement.com/foam-board-insulation-values indicates that:
!) Expanded polystyrene foam (EPS) typically has an R value of 3.6 to 4.0 per inch of thickness.
2) Extruded polystyrene foam (XPS) typically has an R value of 4.5 to 5.0 per inch of thickness.
3) Polyisocyanurate (polyiso) often has a foil facing and typically has R values of 7.0 to 8.0 per inch of thickness.
My teardrop will be insulated with polyiso foam boards to achieve higher R values (which will be reduced by the wood framing):
Floor. The floor will have 3/4" thick polyiso in it. The floor's rigid foam R value will be about R 5.25 to R 6.0. I'll use 3/8" plywood for the top of the floor and 1/8" plywood on the bottom of the floor.
Walls. The cabin side walls will have 1" thick polyiso. The walls rigid foam R value will be about R 6.0 to R 8.0. I'll use 1/8" plywood inside and outside the cabin.
The bulkhead between the sleeping cabin and the galley will have 3/4" polyiso with a rigid foam R value of about R 5.25 to R 6.0. The bulkhead will have 1/8" plywood on both sides.
Roof. The roof and front wall will have 1-1/2" polyiso. The roof and front walls rigid foam R value will be about R 10.5 to R 12.0. I'll use 1/8" plywood for the cabin ceiling and the roof skin.
The outside of the teardrop will be fiberglassed (and painted) to add strength, rigidity and waterproofing.