Thanks for the picture Chris - most helpful. Glad to see I'm not reinventing the wheel here.

Emmett,

Regarding moisture movement through this assembly, I think it would be similar to a SIP if you sealed the seams with expanding foam. My understanding is that the permeability of polyurethane is low enough that you don't require a seperate vapour barrier (provided you seal the seams with foam). I suspect most building inspectors around here would prefer to see a seperate vapour barrier though (6 mil poly).