Skyline Drive just had the underside of the roof sheathing sprayed with 2" of closed cell spray foam.  This 2" will provide an R value of about 13.  Under this, fiberglass insulation with an R value of 30 will be installed.  Together, this will give the roof an R value of approximately 42.  Current California Residential Code requires R30-R38 depending on location and calculation method.  The location of this house only required R30.

Closed Cell Spray Foam completely seals the surface that it is sprayed on.  It not only insulates, but it is impermeable to air.  For these reasons it is often considered the best form of insulation (although this is highly debatable).  It is also the most expensive form of insulation.

Ideally, it would be great to use spray foam throughout the entire house... walls, floors and ceilings.  Unfortunately, as mention above, it is very expensive and doing so would be cost prohibitive on most projects.  To get the best of both worlds, 2" of foam is applied to the under side to the roof.  This seals the roof sheathing.  Then, fiber glass batt insulation, which is relatively inexpensive, is installed under the foam.  Fiberglass batts do not prevent the flow of air and do not perform well in a leaky ceiling, wall, or floor.  Batt insulation, however, works very well when an air barrier is used (foam).  Thus, this combination is very cost effective and results in high performance for relatively low cost.