Jon:
I would put the vapor barrier to the interior surface because you don't want water vapor condensing in the fibreglass insulation (and thus causing rot). The standing seam roof will provide some reflective value for external radiant heat (and I think Englert offers coatings that claim to improve on that as well - depends on whose manufacturing the raw coil for the roof). I am not familiar with the Atlas product but am wondering if it might not make more sense just to stick with rigid foam for your whole roof system. For example: If you use 2 layers of 2" Thermax rigid foam with foil you will get an R value of about R22-30 (I honestly can't remember the final R value but you get around R7/inch plus some bonus for the foil). If you have the time this may save you some bucks. You then are left with some air space beneath the deck for ventilation.
hope this helps,
Andy in NH
