I'm designing a new house and would like some reality checks on the roof stack up; The house in on an island off Rhode Island so technically Zone 5 but the end of long Island is only 20 miles west and that's Zone 4. Winter temps are a bit on the warm side since the ocean tempers it a bit - say a design temp of +5 F. Wind is heavy (120 MPH zone) so air sealing is huge (keeping the roof attached to the walls is too). At $.55/ kWh NO A/C! Here's what I propose: Cathedral ceilings - unvented. From bottom up - drywall with latex paint ceiling finish on strapping on bottom of 2x10 rafters at 16" (span is about 14' max - slope either 10/12 or 12/12). Rafter cavities filled with dense celulose. 5/8 CDX sheathing. 2 layers of 2" XPS with taped and staggered joints. Drainage plane (thinking high perm - like vapro-shield but maybe 30# felt does it too. 7/8 (ripped ledger board) furring on a 45 degree angle and staggered to provide eave to ridge air path. Standing seam metal roof ( with future solar panels clamped to seams). This should give me an R in the high 40's, approaching 50. Walls will be about R-30 - 32 (with 2" XPS outside the sheathing) Questions: Is there a need for a better air barrier somewhere below the drainage plane? Perhaps between the sheathing and the XPS? Is CDX the right sheathing choice? Would Zip board taped give me the air barrier at the same time with the XPS keeping the sheathing interior above the dew point and drying happening toward the interior from the sheathing and to the exterior from the sheathing. (I can't seem to find the perm rating for Zip Panels) Is it even wise or necessary to vent the underside of the Metal Roof? (We have a harsh marine environment here with high humidity, heavy dew and fog. I have a chance to do this right so all comments/thoughts welcome. As always, this forum is fantastic and thanks in advance..... Sam

Are you considering concrete walls or concrete roofs....Insulated concrete roofs can give you cathedral ceilings depend of the concrete form technology....maybe this can help you...www.onevo.com let me know if you have any question about it

I considered concrete - specifically ICF's - but have ruled it out due to cost and concrete quality control issues. As a small island 15 miles offshore we have limited resources for concrete and a yard of 3/4 3000 is well over $200. Admixes and self consolidation is "new technology" here. Bringing a pump truck over on the boat is also hugely expensive. The joys (and costs) of island life......

Yes, I can Imagine.....how much sq.ft you are looking at?

House will be about 2,100 SF.

I'd love to hear comments on this proposed stackup.

Particularly interested in the location of the air barrier, thoughts on Zip board and venting a metal roof.....
Thanks