A-Frame re-roof – what type of (if any) air barrier?
I have an A-Frame (steep pitch cathedral roof) built in 1975. It is an unvented roof. Region 5, southern NH. It needs new shingles. The existing roof from inside out is:
5″ x 14″ laminated rafters 14’ OC
2-1/2″ thick T&G decking
2″ rigid insulation
Asphalt shingles nailed through insulation to deck
Removing the old shingles will harm the existing insulation, and I can find no existing air barrier. The roof is leaking.
The proposal under consideration is:
Remove existing shingles and insulation
Install air barrier to deck
Two layers of rigid insulation (polyiso), stagger and tape all seams
1/2” OSB (nail base), screwed through insulation to deck
Roofing felt of choice
Asphalt (or metal) shingles nailed to OSB
The new insulation will yield about R-26. Low for my region, but 6” of rigid insulation (R39) on top of the deck is problematic. Deck is a strong visible detail from inside the house (no insulation to be added under the deck).
My question is, what should the air barrier be? It has been suggested to use either polyethylene or peel and stick ice shield over the entire deck. There is also an argument that I would not need one at all.
Is the proposal in general sound? Another proposal is to use nail base SIPs and convert to vented. I do not think that is desirable.
Thanks for any guidance.