You just want 100 percent ceiling integrety on the ceiling insulation. Anything more is basically a waste in another vented location. Your attic is ventalated I hope.

Todays Canadian codes amount to about 15 inches of ceiling insulation up here in Canada. You are also far enough north that it makes sense to me as well in your location.

You may want to consider lifting the attic floor or some other solution to increase the ceiling insulation to more modern levels. Chances are the ceiling is just 2x4s. If the house is quite old perhaps a little thicker than current 2x4s. Yet still pretty thin insulation if that is all there is.

The other option is to finish the room up here and insulate it correctly. If the sidewalls are already high enough in would be pretty easy to put a collar tie across the attic space at each rafter.

The only issue is it is somewhat less effective than just increasing the existing attic thickness as you have increased the volume of the house as far as heating goes to some extent.

Also make sure you put extenders up the eave slots so the soffit vents do not get obstructed with new insulation on the ceiling. I guess you could just spread it over the existing subfloor in the attic as well. Depends on how you want to deal with the storage space.