Shed roof metal roofing venting

A homeowner in New Hampshire is building a shed-roof addition and asked whether the contractor's planned roof setup makes sense. The plan called for pine ceiling boards on the rafters, 8 inches of rigid foam insulation (polyiso), two layers of strapping to create a vent space, plywood-style sheathing (Zip), a peel-and-stick waterproofing layer (Grace ice and water shield), and a metal roof on top. The roof has no cathedral-style ventilation gap built in above the living space.

A building science expert who replied pointed out a problem: when a roof has a continuous layer of foam insulation on the outside like this, it's considered a sealed, unvented assembly. Adding a vent space above the foam isn't needed and can actually cause trouble, since it gives moisture a place to enter and condense rather than a path to escape. The suggested fix was simpler: tongue-and-groove ceiling boards, a self-adhered weather barrier acting as the air seal, taped foam board, roof sheathing, a synthetic underlayment made for metal roofs (if using plywood sheathing), and the metal roofing on top. He also stressed sealing the ceiling boards where they meet the exterior walls, to stop air leaks.

For anyone building or re-roofing a similar shed-style addition with rigid foam over the rafters, the takeaway is that skipping the vent layer and focusing on airtight seams and a solid air barrier is the more reliable approach.