Unvented cathedral ceiling (CZ-3B, spray foam alternatives) — feedback – GreenBuildingAdvisor

Hi all,

Looking for a sanity check on a cathedral ceiling rebuild after water damage. I’ve read extensively on GBA and want to confirm I’m not missing any moisture risks.

Context

• Los Angeles County, IECC CZ-3B
• Cathedral ceiling (no attic), 2×6 rafters 12 inches on center
• Retrofit/repair, not new construction
• Some irregular bays due to sistered rafters
• One skylight above shower (hoping to keep – but could remove to reduce drywall air sealing risks)
• Preference to avoid closed cell spray foam (potential health risks)
• Rigid foam above roof deck seems difficult, due to complicated roof geometry (house is “T” shaped, and only one half of the house has unvented cathedral)
• Priority is moisture robustness and drying

Proposed assembly (interior → exterior)

• Airtight drywall ceiling (taped/mudded; careful at penetrations)
• R-21–R-23 fiberglass or mineral wool in rafter bays, full contact with sheathing
• Plywood/OSB roof sheathing (taped where feasible)
• Vapor-open synthetic underlayment
• Shingles (standing seam is also an potential option on the table, on a batten grid)

Rationale

Unvented rafter bays with air control at the ceiling plane, (very limited…but not zero?) outward drying via vapor-open underlayment and shingles. In CZ-3B, my understanding is this avoids the need for ccSPF or exterior rigid foam if drywall air sealing is good.

1. Is this assembly sound as described for CZ-3B?
2. Any concerns with air-permeable insulation at the sheathing? I understand this would not fly in many climates, where cc spray foam would be a primary option, but it may be allowed here in 3B.
3. Is a vapor diffusion port something to be considered here, and would that change in regards to standing seam vs shingle?
4. Should I just bite the bullet and go with spray foam?