Ventilation could be the most important factor in a cold roof system.  Too much, and your roof will not be able to create a draw and maintain constant airflow through the system.  Too little, and your roof will not be able to move the warmer air through the system fast enough to maintain a cold roof deck.

When understanding ventilation, we need to first start with how much air we are allowing into the system, typically through the soffit.  Next we want to determine how much air is moving through the ridge vent.  The idea is to create a draw from the eave to the ridge just below the roof surface, as the surface of the roof begins to heat from the sun or from heat within, it will naturally want to rise to the ridge, pulling cool air in from the eave.  If you do not have adequate eave or ridge vent, the system will be constricted, allowing the roof surface to heat up from the sun or heat loss from the building.  Another factor that can effect roof ventilation is the roof pitch, the steeper the roof the easier it is to create a draw.

Insulation – Ventilation – Roof Pitch – Roof Design & Layout