I-Joist 2:12 roof – 1.5” air gap enough? – GreenBuildingAdvisor

We’re building a 2:12 shed roof running north/south, with no penetrations, hips, valleys, etc., and are using 16” I-Joists. It’s all cathedral ceiling.

I’d like to simply attach plywood/osb to the upper I-Joist block to make a 1.5” ventilation channel, venting the roof fully from soffit to ridge.

With a tight ceiling and I-Joist cavity filled with R-49 fiberglass batts (can’t find anyone who will dense pack cellulose), is a 1.5” ventilation gap enough, or do I need to go unvented with rigid on top of the roof deck instead?