Rockwool exterior insulation with metal roof

This is a forum question from a homeowner building their own house, not a news story, so there's no program or policy update to pass along. A builder posted on a building-science Q&A site asking how to finish a shed-style ("monoply") roof. They've already installed sheathing and want to add 4 inches of rockwool (a dense mineral-wool insulation board) on top, held in place with 2x4s acting as both structural support and an air barrier, then a metal roof over that. A metal-roofing company told them they'd need a layer of OSB board on top of the 2x4s first, but the homeowner wants to skip that second layer of sheathing to save money, weight, and labor since they're working mostly alone. They asked whether horizontal 1x4 strips (purlins) under corrugated metal panels could work instead, and how to keep water from getting trapped under the metal on a sloped roof. The house is in climate zone 4C, and they hope to close in the roof before winter.

If you're planning a similar exterior-insulation retrofit on your own roof, the core questions here are worth keeping in mind: how the metal roofing is fastened down without a solid deck, and how water and moisture move through the insulation layer. These details affect long-term durability more than they affect energy performance, so they're worth getting right before committing to a specific roof assembly.