Too many vapor barriers with a cold (unvented) roof retrofit?
I’d like to retrofit our existing ventilated roof to an unvented (cold) roof per Martin Holiday’s musing “How to Build and Insulated Cathedral Ceiling” (https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling) and various Building Science articles but specifically the one included in Fine Homebuilding recently. (http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting) In the Building Science article Joe Lstiburek provides details on how an unvented roof can be built by insulating above and below the roof.
Our house was built with a ventilated roof with continuous soffit and ridge vents as well as kraft-faced fiberglass installed under the plywood roof deck and fiberglass composition shingles. I don’t believe that any baffles were installed between the fiberglass and roof deck. Many of our ceilings are cathedral but we do have some attic space with the exposed kraft paper of the fiberglass batts above.
My question is about the retrofit. I’d like to add foam, and a metal roof above the existing roof after stripping the shingles per the specs in the article referenced above and shown in the included drawing from the Building Science paper. I’d, of course, seal off the soffits and remove ridge vents. I’d much prefer not to have to remove the existing plywood roof deck.
I have two primary concerns.
1) Would the combination of removing ventilation and having two vapor barriers — one, the foam and roofing membrane; and two, the existing kraft paper — cause moisture-related problems in the fiberglass insulated area between the rafters?
2) Will the air space between the existing fiberglass and roof deck be a problem?
My apologies if this has been covered in another post. I searched but didn’t find anything on this specific issue.
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