“When we do complex roofs in high snow load areas like ski resorts we typically have the roof deck act as the air control layer and provide a high level of thermal resistance at the roof deck (either rigid insulation directly above the roof deck or high density spray foam directly under the roof deck – and then we provide an over roof creating a vented space over the top of the entire assembly. In essence we created a vented unvented roof hybrid. An unvented primary roof assembly with a vented over roof.”
from comment#53 here:
Lstiburek’s Rules for Venting Roofs
You need an airtight ceiling, lots of air flow, plenty of soffit vents, and deep insulation at the attic perimeter
Awesome awesome article on insulating roofs.