Going to have to go back and add your location to your profile.
Must be missing something, of this is not a finished attic why would you insulate the rafters?
Should be insulating the ceiling to seal the conditioned envelope not the roof.
Ice dams are caused from heat escaping the ceiling and outside wall area and lack of venting of the roof.
An unfinished attic need to be close to the outside temperture.
http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table
4" sure sounds thin anyplace in the US.
Must be missing something, of this is not a finished attic why would you insulate the rafters?
Should be insulating the ceiling to seal the conditioned envelope not the roof.
Ice dams are caused from heat escaping the ceiling and outside wall area and lack of venting of the roof.
An unfinished attic need to be close to the outside temperture.
http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table
4" sure sounds thin anyplace in the US.