Overall, your plan is a sound one. However, 2" of rigid insulation is not much R-factor. I don't know if you are planning to use EPS or Isocyanurate, but assuming that it's the higher R ISO (at about R-7/inch), you'd only end up with maybe R-19 after you add in the other benefits of the deck etc. Thus, I'd recommend at least 4" in the floor and maybe even 6" in the ceiling. There is no need for the 2" to 4" of ventilation channel that you mention. It is only necessary to insure that the cross sectional area of the channel is equal to the inlet area and the exit area at the ridge. From my experience this can be easily accomplished by installing cheap 5/4" strapping on 16" centers to provide the channel. Because your size is small, there won't be much savings from installing rigid foam to avoid the expense of stresskins. So, you might consider just purchasing ISO panels faced on one side with OSB and then nailing the strapping onto the OSB (outer) side.

Good luck,


Brian