2" is best, but not in a hot/humid climate;
"In vented cathedral ceiling assemblies a minimum 2-inch clear airspace is recommended between the underside of the roof deck and the top of the cavity insulation. This is not a code requirement but ought to be (only 1-inch is typically specified in the model codes). It is the author’s experience that typical installation practices and construction tolerances do not result in an airspace of at least 1 inch and rarely is it “clear.” Even when 2” clear space is provided, the rate of ventilation flow will be significantly less than in an open ventilated attic."
"In hot climates, the primary purpose of attic or roof ventilation is to expel solar heated hot air from the attic to lessen the building cooling load. The amount of cooling provided by a well ventilated roof exposed to the sun is very small. Field monitoring of numerous attics has confirmed that the temperature of the roof sheathing of a unvented roof will rise by a few to no more than 10 F more than a well ventilated attic." From; http://www.buildingscience.com/docum...on?full_view=1
Test results show increasing the ventilation rate from 1/300 to 1/150; pp.7---- almost no difference in shingle temps; pp.8----4*F difference in plywood sheathing temp., pp.10--- less than 1% annual net effect for heating/cooling; http://www.buildingscience.com/docum...n-hot-climates
From pp7; " With 1:300 vent area, adding the radiant barrier system
reduced the ceiling flux by 26%. Increasing the vent area to 1:150 improved the reduction to 36% (an additional 10 percentage point reduction) while increasing the relative humidity in the attic by
6-10%. The average humidity without the radiant barrier system and with 1:300 was 53%, when the radiant barrier system was added the average was 54%, and when the vent area was increased
to 1:150 the average relative humidity increased to 62%." From; http://www.google.com/url?sa=t&rct=j...mH81PA&cad=rja