Under roof deck venting with 'cathedral' ceiling – GreenBuildingAdvisor

I’ve been trying to determine how to insulate and ventilate my 2:12 slope ‘cathedral’ roofs in climate zone 6a with 70psf ground snow load for a near-passive level house design. My structural engineer has specified 24″ deep parallel chord truss due to the 28’3″ span on one side and applied it to the other slopes for consistency. There will be no foam used for this project for reasons I’ll not get into. My latest thought was to have a standing seam metal roof over plywood decking which is covered with Siga Majvest 200SA, then use “SmartBaffles” between the top chords of the 24 o.c. trusses to provide a 3″ or more ventilation gap up the entire roofline. A metal flashing detail will allow vented air to escape before any intersecting vertical wall. The rest of the truss will be dense-packed with TimberHP TimberFill, with Siga Majrex on the underside of the trusses (over the netting for blown-in), followed by 2×4 strapping perpendicular to the trusses, and then drywall. The house will be airtight down to .25ACH at 50 pascal and a dehumidifier keeping the house around 20-30% RH. I’ve not been able to find this setup on GBA, so there’s undoubtedly a good reason for that. Thoughts?