Probably too late to be asking this, but I had my garage attic converted from Purlins to a Knee wall and into a semi conditioned space.  The knee walls are 2x4 and the existing rafters are 2x6.  I am in the process of putting 3/4" rigid foam on the outside of the knee wall and faced R-13 between the 2x4's with the faced side towards the room.  I initially stapled R19 Batts between the roof joists that were of all different dimensions.  I had a local guy tell me I needed to pull it all down and put baffles in there against the LP Techshield sheathing because it would get moldy and rot.  I told him I considered open cell foam too, and he said that would also cause it to rot because of no moisture barrier.  However upon reading on the internet moisture seems to be more of an issue in colder climate resulting in rot of the roof deck in cold climates.  These baffles because of the different size cavities between the joists are somewhat of a pain to install.  Also because it is a hip roof some of the cavities never even make it to the ridge, so as far as air movement, it is limited between some of the joists.  The span at the peak inside the room is about 84"x 81".  I took some of the 3/4" rigid foam and nailed it above my head to the new joists they put in when the purlins were removed.  I can't really get much insulation above that foam now as I bet there is only about 10 inches from the ridge of the roof to the top of the foam now.  Another problem I am facing is that some of the R19 (because of the odd width sizes between the roof rafters) is sagging so, when I either a) sheet rock it or b) put rigid foam on their then sheet rock it, it is going to compress the Batts significantly reducing the effectiveness.  I am not trying to live in this space, just trying not to melt my wifes Christmas decorations and other crap.

Are/have been the rafter baffles been a waste of money because of the hip roof?  Right now there is no ridge vent, the ridge is about 3 to 4 feet, should I add one?  Will the (compressed) R19 ever be enough to keep it below 100 degrees up there?  It seems for cathedral ceiling in southeast Texas you would need like 2x10 or 2x12 roof joists.