A good friend of mine is having his roof redone soon and would to insulate it. The assembly from seems to be 2×6 ceiling joists with 2×8? rafters above that. 

What we know..
A few rafters need to be replaced or sistered accordingly. 
2×6 ceiling joist  + spacing between joist and rafter + 2×8? possible rafters
4 Ventilation openings , 2 On the north side and 2 on the southside. 

Since he will be replacing the sheathing we will have unrestricted access to the ceiling below. 

My original plan was to box out can lights and purchase a spray foam kit and air seal and spray 2 inches topside of the drywall then blow in cellulose or rockwool up to vents. 

option 2 was to caulk and spray foam all cracks and gaps in addition to boxing out cans, assuming drywall is a sufficient air barrier with proper sealing topside. blow in insulation in the same manner. 

option 3 would be to blow in insulation up to top of 2×6 then staple down an air barrier such as intello plus or  6 mil poly. Followed by some more blow in insulation up to vent openings. 

All 3 would be superior to what he has now. I remember reading here that cellulose is a pretty good air barrier on its own with ample amounts. I this plausible just blow in 10-20 inches and call it a day? 

Also the vent opening seem a bit frugal to me. For proper air flow and to truly keep this a cold roof will I need to add a few more? 

Just preliminary thoughts but eager to hear from the community on this one. 

Btw we are in chicago climate zone 5

Raul