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Is vent over exterior roof insulation worthwhile?

stolzberg | Posted in General Questions on

I asked this about a month ago with no response, wondering if anyone has an opinion on the value of venting above exterior roof insulation over conditioned attic. I’m looking into replacing a slate roof in northwest Massachusetts (zone 5), thinking about adding rigid foam insulation over original board sheathing, then layer of 1/2 inch plywood sheathing to support synthetic slate tiles or metal roof.  Is it worth adding vertical 2x4s on flat on top of the rigid foam and under the plywood to create soffit to ridge vent channels over the insulation.  It’s a steep roof (50 degrees).  Here’s the details under consideration from outside in:

1. roofing: metal or synthetic slate
2. underlayment
3. 1/2 inch plywood
4. flat 2x4s running vertical from eave to ridge forming vent channels
5. 7″ polyiso
6. peel and stick membrane as air barrier
7. original board sheathing
8. real 6 inch rafters filled with fiberglass batt insulation
9. shiplap for attic ceiling

The roof has two giant dormers, which would limit air flow from soffit in those sections.

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