How can I insulate my attic in southern California?
If it was originally built correctly, the cathedral sections should already be adequately insulated...  There is actually a vapor permeable membrane that is used between the decorative side you see indoors and the functional side your roofing is attached to.  While this insulation is relatively thin, it serves the purpose as an air-tight barrier.  The rest of the R-value (in essence) of this style of ceiling is known as thermal mass, where thicker wood or material is used to prevent temperature fluctuations.  For some references to answer your questions in more detail see:  http://www.applegateinsulation.com/Product-Info/Technical-Pages/249234.aspx or https://web.archive.org/web/20120629164116/http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11430