Vented “over-roof” sheathing
I’m building an unconditioned workshop with a cathedral ceiling (i.e., no attic) under a shed roof.
The planned roof stackup, from inside to outside:
Exposed TJI Rafters @ 24″ OC
1/2″ plywood sheathing w/ h-clips
fully adhered WRB, continuous with wall WRB
2 layers of 3″ of Roxul ComfortBoard (R-24 total)
2×4 purlins parallel to ridge, connected to rafters via long screws (these purlins provide a ventilation channel to prevent ice dams)
Snap Lock metal roof (e.g., https://www.pac-clad.com/metal-roofing/snap-clad/) attached to purlins
My question is whether I need an additional roof deck + underlayment on top of the insulation.
Drawings of “vented over-roof” assemblies:
show this layer, but I don’t understand why it was added.
Is it an air barrier on top of the insulation to prevent windwashing?
Assemblies using rockwool as exterior wall insulation w/ a rainscreen gap don’t seem to have windwashing problems (e.g., https://www.greenbuildingadvisor.com/blogs/dept/musings/installing-mineral-wool-insulation-over-exterior-wall-sheathing)
Is it a water barrier?
Why isn’t the fully-adhered WRB underneath the insulation sufficient?
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