Ventilation IS necessary, albeit maybe less so if you have a slate roof that “breathes” towards the outside (compared to torch down and other “sealed types”).
If I were in your shoes, I would create soffit vents somehow, make sure you get 1"-1.15" of space between underside of deck and insulation (regardless of what kind). In order to increase the “depths”, furr out existing 2x6" with a 2x2" or a 2x4" screwed and glued in place. It both allows for added space as well as stronger frame members. But, before doing this, allow the lumber to dry inside your house so it reaches a humidity level that is comparable to what is in the rest of the frame lumber.
I have been working somewhere in the residential energy services/consulting/inspecting fields since 1977 and have seen thousands of 1.5 & 2.5 storey cathedral style steep roof cavities that were blown full with rockwool, fiberglass or (my favourite) cellulose. Most of these homes were blown in the 1950’s-1980’s. These roof slopes did not show signs of deterioration by moisture. Why? They did not have many or any holes in the sloping ceilings to let warm house air containing moisture into the cavities and the houses and due to general air leakage had fairly dry air. We can reproduce similar conditions in new homes/renovations as we now have the knowledge and techniques to prevent air leakage into wall/ceiling cavities by using air barrier systems and we can control interior moisture levels by cleaning up moisture problems and installing proper interior house ventilation.
There has been a movement afoot for 10-15 years to allow unvented cathedral cavities and roofs that has been recognized by some major building organizations incuding code producing orgs and even some roof shingle manufacturers.
https://www03.cmhc-schl.gc.ca/b2c/b2c/init.do?language=en&shop=Z01EN&areaID=0000000026&productID=00000000260000000011
http://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates
If you plan on using fiberglass batts, make sure the ventilation baffles does allow the insulation to “breathe” into the moving air. Stay away from “Durovent” or other type of manufactured baffles out of styrfoam. Water vapor will only condensate on these baffles and eventually soaking the insulation. Custom made baffles out of “peg board” or some type of “mesh”/netting would be better. Only purpose is to hold insulation away from roof and maintain the needed air space.
This is a scare tactic!! This may happen if the house is run at very high humidities **and **the vapour retarder/barrier mentioned below is simply overlapped and not sealed to be the air barrier now required by Canadian codes (since 1985-1990)
Needless to sya, do not skip on the vapor barrier. Do NOT trust the paper on the insulation. Put on a proper PVC or similar type barrier with good overlaps at the seams.
From the former American Plywood Association (APA), now The Engineered Engineered Wood Association: http://www.contractorsinstitute.com/downloads/Window,%20Door,%20and%20Wall%20Flashing%20Details/APA%20Window%20Flashing.pdf
See pages 9-10: “While polyethylene sheeting makes a very good vapour retarder, it is relatively difficult to install. In most cases, the use of polyethylene is not necessary, even in very cold regions. Ordinary interior latex paint applied over drywall can provide sufficient vapour retardent properties.”
This last statement has also been said by researchers at the Canadian National Research Council, The US Corps of Army Engineers, Building Science Corporation (BSC), Advanced Energy Corporation.
From Can. NRC Building Digest #175 (March 1976):
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd175_e.html
*“Air leakage is now considered to be the prime cause of most condensation problems in walls and roof spaces. If, therefore, a building can be made tight against air leakage it may not need a vapour barrier, as defined. On the other hand, if there are openings that permit air to leak from the warm side to the cold side of the insulation, adding a vapour barrier (even of zero permeance) that does not seal off the openings will be useless.” *
From a BSC document:* http://www.buildingscience.com/documents/digests/bsd-104-understanding-air-barriers/view?searchterm=vapor%20retarder*
"Keep in mind however, sheet polyethylene on the inside of building assemblies in cold, mixed-humid, marine, hot-dry and hot-humid climates is not generally a good idea; drying of building assemblies in these climates occurs to the inside as well as to the outside.
Interior drying is typically necessary in air-conditioned enclosures. In other words, interior vapor barriers such as polyethylene and vinyl wall coverings should never be installed in air-conditioned buildings – even ones located in cold climates."
The more I learn about American roofing systems, the more I am in favor for clay or concrete tiles that allow the roof to properly breathe. In (native) Sweden, there are very few problems with these roofs, in rain, or in snow, and they last 50+ years. Where I can see them less suitable are in eartquake prone areas since these roofs are very heavy (but still pretty common in CA).