I’ve seen a few discussions on this, but wanted to get some feedback on my particular issues. I’m an owner-builder with some construction experience, but based on all the comments on this site,  this seems like a complex subject

I’m doing a major re-model, but basically new construction. I’m in So Cal,  which in cat 3 I believe. Im subject to Title24 that came up with the required insulation values. I am planning a high reflective metal roof on the entire structure with  stick down, high temp underlayment and 5/8 CDX sheathing.

In one section of the house, I have a 2×10 24OC vaulted ceiling that needs R38.  I was planning to use R20 Closed Cell(CCSPF)  and then R19batts.  This should fit in the 2×10 and allow me to  more easily do can lights etc.  Also, cheaper than all CCSPF. Can I assume that the R20 CCSPF is a good enough vapor barrier? Should i be concerned about this in So Cal?

In another section, a 1200sq ft addition, I have a 4:12 slope roof. Heat Pump AC/Heat unit and ducting in the attic.  I will have a IAQ fan in this section.  T24 says I need R8 in the roof and r38 in the ceiling.  I was going to do r8 ccspf directly between the 24OC 2×4  trusses and r38 batts in the ceiling.  After reading some articles and comments, I wonder if it may be wiser to do all the insulation under the roof to change the attic to a conditioned space.    I was thinking either  r8CCSPF + R38 batts or go R38++CCSPF or maybe R8CCSPF +R38OCSPF   I am expect(hoping?) that the SPF would provide a vapor barrier and  would eliminate the need to provide ~60sq ft of attic ventilation…I hate putting more holes in the roof than necessary.  This , along with energy efficiency of having HVAC in a conditioned space would be the primary motivation to put all the insulation under the roof.

Thanks for any advice.