Rebuilding a 1995 bungalow north of Toronto in Canada.  Roof structure is a parallel chord cathedral truss.  Will be R40 batts between the trusses a 2 foot centre.  

Roof is vented

Roof assembly will built prior to any interior walls.  Current plan is insulate and smart vapour barrier and the strap for service cavity and the drywall. 

Has anyone added a layer of continuous foam insulation.  Foam could be vapour barrier and eliminate the thermal bridge at each truss.  

Thus assembly would be truss, 2” of foam taped at joints, R8-10 depending on brand, 2x strapping and drywall.  

Thanks