Upstairs Partitions, Underslab Insulation

Got delivery of a load of 2x4's to frame the walls for the rooms on the second floor, and started nailing them together to finish the day today. The same delivery brought a pile of 2 inch polystyrene insulation to go under slab. The ground on the first floor is groomed smooth and level, and it's ready for vapor barrier, insulation and wire mesh. I've been going back and forth about the virtues of different insulating systems, and I've settled on the option that yields the biggest bang for the buck but requires more effort on my part. A lot of my building decisions fit this formula; my time, while not free, comes pretty cheap, and I'm sometimes willing to take on a time consuming process if the benefit is commensurate to the effort. In this case insulating with fiberglass batts and installing a thermal break to the inside would be the least effort option. But I don't really trust the effectiveness of fiberglass, and the R-value with a flawless installation would be around 27. The option of greater effort involves getting a shipment of 2 inch polyisocyanurate (polyiso to those in the trade) by tractor trailer and cutting it to fit the space between studs. I'm sure I'll come up with a quick and easy way to cut the stuff to size. Two layers of that plus a thermal break to the inside would yield an R-value of somewhere in the low 30's, tight to the inside and breathable to the outside.