Dana - thank you for the thorough response. Your answer gave me a few more questions though...

1 -

It's important to use an air-tight underlayment between the EPS and the t & g roof deck for overall performance reasons

- Would a sheet of Tyvek house wrap do? What type of product do you recommend? Also, isn't the EPS an air barrier? I as going to seal all the edges and corners with great stuff or something similar.

2 -

A better solution for zone 5 is to install a smart vapor retarder (Certainteed MemBrain or Intello Plus) between the interior gypsum and the cellulose, and design in a "rainscreen" gap between the exterior sheathing and the siding

I was planning on an exterior rain screen, but are you saying I wont need an interior vapor barrier at all? And rather just a retarder?

The EPS is in sheets with seams that need to be taped, and lower density EPS isn't all that air-retardent. EPS expands & contracts with temperature, and seasonally the seams may open & close. Taping the seams is worthwhile, but it's not a primary air-barrier.

Housewrap would be a LOUSY underlayment between t & g sheathing and rigid foam. Ideally you'd want it to a rugged slip surface to accomodate the differences in seasonal expansion & contraction of the wood vs the foam. People have successfully used a number of different products, but sometimes the self-adhered goods can become a problem.

Yes you don't need an interior vapor barrier, or even a vapor retarder, as long as it's truly air tight between the roof decking and the foam. Interior vapor barriers are only needed when the material above the ceiling is air permeable (like fiber insulation) in combination with insufficient venting to purge any moisture diffusion to the outdoors. On the cold side of US climate zone 7 and into climate zone 8 it's worth it even on vented fiber-insulated attics, but there's no point to it if all the insulation is EPS, even with modest venting above.