Ice dams are very much related to condensation problems. What causes ice dams? Temperature right under the roofing cover should be as close to outside temperature as possible. No excuses that building design does not allow having that. How can we accomplish that?
Insulate and isolate
Eliminate moisture pumped into the attic by bathroom vents, kitchen vents etc.
Ventilate remains of heat and air vapor. But how, when we are dealing with dormers, roof valleys, skylights, can lights, cathedral ceilings, luck of soffit overhang and a combination of all of that. This often requires lots of creativity right on the job site.
Alternative approach in specific situations: insulate well and disregard ventilation, however be aware that potential air vapor may penetrate through the gaps to roof cover area.
Other alternative approach: heating cables often being installed for two major reasons (1) far less expensive alternative in specific situations; (2) this is the only way to resolve certain issues.