The first thing you need to decide, is not how to ventilate the space, but whether to ventilate the space. Depending on your local climate, temps and humidity, you might be best off sealing the space. Check on some building science web sites about sealed attic systems. In addition post back here with some details about your climate. Lastly, if you are planning significant changes to your home over the next few years, consider hiring an energy rater to look at the whole house to identify the low-hanging fruit for energy savings in conjunction with your projected renovation and repair needs.

Since you're taking the roof off right down to the rafters (I assume the 1x6 decking is too rotted to keep, thus the plywood ) you have a lot of options. You can use SIP type panels over the entire roof. You can use high density foamed in place insulation on the exterior side. No doubt there are other options as well. Ventilation system depends on the insulation type and installation. Well worth getting several estimates/proposals.

Thanks for the great replies.

I should have mentioned the climate. I'm in Western Oregon (Portland) so overall things are pretty mild here most of the time. We typically get a week or three of 90-100 degree weather in the summer and maybe touch the high teens every few winters with little snow. While it rains a lot we basically never get hot and humid conditions like you see in some parts. We're apparently "zone 4c".

Since we have the finished attic (where we sleep) I assumed that ventilating will be a benefit in the summer when the sun tends to bake the roof and make it pretty hot upstairs, but that's open to discussion.

@maingrower - no, the existing decking seems to be in good shape, but 2 out of 3 roofers want to replace it anyway. I'm not clear on why. SIP panels look interesting, as does the spray foam... but how does the roofing go down on top of that? Another layer of decking?

Any suggestions on relevant Building Science sites? What I've found on the first pass seems mostly relevant to new construction.

Thanks again!

JustTad: If the decking is in good shape, I don't see why the roofers want to replace it or put plywood over it. You might have to provide ventilation between the deck and SIP panels should you choose to go that route.

A friend did the exterior foam option just a few years ago. There was a historically valuable plaster ceiling in the rooms below, so increasing insulation from the exterior was the best option. As I remember, 2x4's or 2x6's were put on over a membrane which was put down over the original deck. The area between the 2x4's was filled with high density foam. Another deck, then cedar shingles. I can't remember anything about the venting.

I worked on a some condos that had vaulted ceilings and no overhangs. They actually put the round style 3" vents in the wall in the center of each bay and then covered it with a trim board that went under the drip edge. It didn't seem very efficient to me but maybe a few more vents and some spacers for the trim and it could be applicable.