ventilating closed in rafters

What is the proper way to ventilate closed in rafters. I am building a new house and the "great room" has cathedral ceilings made with 16" I joists on 24" centers at a 14/12 pitch. It is sheathed with 6" boards then tar paper and asphalt shingles. The boards are fairly green being sawn this past summer. Anyway I was told to use styrofoam truevents top to bottom (~18 feet) so air can enter the bottom and exit the ridge vent. As I see it, this will not draw moisture out of the space with insulation. Should I space each 4' true vent from each other by an inch or so? Should I cut little holes periodically. I am just thinking that if moisture finds it way through the vapour barrier and into the insulation it should have a way of getting into the space provided by the true vents. Also the true vents I have seen are only 12" wide and the rafters are on 24" centers. Should I put 2 side by side or is 1 enough. Thanks for any opinions.

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