As I read Josh Salinger’s article in the July issue I’m questioning the installation of the WRB on top of the roof rafters to create a 1.5″ air gap for the roof ventilation. 
In my experience with dense-packed cellulose, the WRB would bulge out and probably restrict the area of the air channel, maybe with 75%.
With the roof sheeting in place before the cellulose installation, there is no way to “belly roll” the WRB to create the full air channel.

I would like this insulation system on an upcoming project but I’m not sure if I would have sufficient ventilation. 
Any feedback out there?