I’m looking for recommendations on the best way to insulate a cathedral ceiling in a hot & semi-humid climate (I live in San Antonio).  The space I’m insulating is a detached garage where half the space is a garage and the other half of the space will be a conditioned living space (aka my workshop).  Right now, I’m thinking of using closed-cell foam on the ceiling of the conditioned space & Roxul on the sides.  I won’t insulate the garage side but would probably add a roof vent or two and a few vents at the eaves just for air flow.  Is this a good approach?  Or should I go with a vented roof and install a ridge vent and eave vents on both sides, and use Roxul + some sort of air channel?

Here’s some additional information:
– Detached one-story structure is 24×27 (with each side about 12×27)
– The structure is about 80 years old, but the entire roof is new (including rafters, sheathing, and shingles)
– The roof pitch is about a 4 or 5
– The roof does get a lot of direct sunlight (no shade trees)
– Conditioned side of structure is on pier & beam with hardwood floors
– There’s currently no roof venting (i.e. no ridge vent)
– There’s a wall separating each half that extends all the way to the ridge (I plan on insulating this wall)
– I’ll be using a wall air conditioner (which will be set around 85 degrees when the space isn’t in use)
– Most of the space doesn’t have house wrap (the west facing side does because that was redone)
– The building has wood lap siding
– And keep in mind that although I’m using the conditioned space as a workshop, it may be used as a tradional living space at some point