Not trying to reinvent the wheel here, I followed in the footsteps of the van sherpas on here and went with 1/2″ Polyiso and 1/2″ Baltic Birch. When I got home with the Birch, I was chastised by my girlfriend’s father (who is much handier than I and tends to know things) for using such nice cabinet wood for a floor that’s going to be covered. My ego couldn’t take the hit, so I returned the Birch and got nice 3/4″ AC plywood. Back in his good graces, I continued onward with $70 more dollars in my pocket for future fancies.