Thanks traveller for the pdf. The pdf basically hedges, I think -- it states in the retrofit section that you MAY want to blow in insulation, but you also want to preserve ventilation. And it points out the mansard part doesn't need ventilation.
A couple facts: I don't have condensation/ice dam issues. And (unfortunately) I do not have existing soffits where the roof meets the steeper mansard. There just isn't an overhang to install them.
I spoke with my inspector and a roof guy who knows my house. Both guys recommended to NOT blow insulation into the mansard part of the roof, but just keep the 4 ft baffles in place as I have them (installed where the mansard meets the roof), and blow-in insulation in the attic.
My plan is to
1) blow in insulation into the attic
2) see if it makes a difference
3) if it continues to be cold on my second floor, then I'll get some infrared photos to see where the thermal break is. If it's at the junction of the mansard and the roof, I'll consider blowing in insulation into the mansard part of the roof.
Regarding ventilation, I think I'm at a loss as far as options go, short of redoing the entire roof structure. It sounds like soffits on the under eaves is silly; and I can't do ridge vents or add soffits where the mansard meets the roof as there isn't an overhang.
My inspector says 2 gable vents & 1 attic fan are decent ventilation for a 1000 sq ft attic. (The gable vents are ~8 sq ft total). My roof installer says that is woefully inadequate. Although they disagree, I still think that I don't have any options to improve it, short of redoing the entire roof -- both of them agree on that.
That's the plan! Will check back for posts, and will reply after a couple weeks with the new insulation. (going from 5 in cellulose to abour 24 inches).