WW is correct, install drywall as the air barrier.
From what I understand, Zone 5 is the cut-off point whether or not to vapor barrier an assembly (ventilated without exterior foam board). General guides from BSC:
“8. Zone 5 requires a Class III (or lower) vapor retarder
on the interior surface of insulation in ventilated insulated roof or attic assemblies.” From: http://www.buildingscience.com/docum...vapor-barriers
“In addition to an air barrier at the ceiling line, a Class II vapor retarder (see sidebar) should be installed in Climate Zones 6 or higher
(see Map 1
Class I vapor retarders (i.e. vapor barriers – see sidebar) can be installed in vented attic assemblies in Climate Zones 6 or higher
(see Map 1
) but should be avoided in other climate zones as top side condensation can occur in summer months during air conditioning periods.
No interior attic assembly side vapor control is required or recommended in climate zones other than
Climate Zones 6 or higher (see Map 1
) for vented attic assemblies (note the distinction, this is not the case for unvented attic assemblies as will be discussed later). With vented attic assemblies moisture that diffuses into the attic space from the conditioned space is vented to the exterior by attic ventilation.” From: http://www.buildingscience.com/docum...on?full_view=1
Your roof is similar to a slab or a wall—fig.#4: http://www.buildingscience.com/docum...e-perfect-wall
Using a link from the end of that article, in your Zone 5, you have similar HDD to Chicago or Boston. Notice fig #3a; http://www.buildingscience.com/docum...nd-wall-design
The drywall is enough, another low perm vapor retarder/barrier as vapor barrier paint (0.45 perms) on one side and plywood (0.75 perms) air channel would give you a v.b./r. on both sides of the assembly. Not recommended for your Zone.
Be sure to air seal the drywall as WW said. This will stop more moisture potential (-100 times more as page 10 shows) than by diffusion; http://www.buildingscience.com/docum...wall-approach/