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What water/air barriers in this roof assembly

stolzberg | Posted in General Questions on

We’re thinking about replacing an old slate roof over gapped board sheathing and real 2×6 rafters in northwest MA (zone 5). With the following roof assembly, what affordable yet effective water/air barriers and/or underlayments would you use and where would you put them?  From the outside in we’re thinking:

1. Metal or synthetic slate (or asphalt shingles if need be)
2. 1/2 inch plywood
3. Vertical or diagonal 2x4s  laid flat to create vent cavities
4. 5.5 inches of polyiso rigid foam (2 layers taped)
5. Original board sheathing
6. Fiberglass batt insulation between 6″ rafters
7. Shiplap

I was thinking some sort of WRB between the board sheathing and rigid insulation and some sort of underlayment between the plywood and whatever roofing material we choose.  I was also thinking the WRB would serve as the air barrier plus taped foil faced polyiso as back up. Wondering what should be permeable and/or not permeable?   Thanks for any advice.

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