Recent attic I inspected had all the insulation removed and the rafters were encapsulated with open cell spray foam.
If the attic or crawl space has an opening that can be entered then the spray foam needs to have an ignition barrier. This barrier can be 1/4 plywood over the foam insulation.
In a typical attic with hatch access this would allow for the 2x4 rafters to be filled giving 4 inches of insulation between the roof decking and the ignition barrier.
Open cell Spray foam has an Rvalue of 3.7 per inch giving your newly sprayed attic a 14.8 Rvalue.
Depending on what climate zone map or insulation requirements you look at. Attic insulation in zone 4-6 should be Rvalue 30-55.
Required thickness needed to achieve this would be at a minimum:
Open Cell @ 8 inches thick
Closed Cell @ 4.5 inches thick
the minimum Rvalue requirements for an attic that can be entered can not be achieved by spray foam alone.
would you then recommend additional insulation (blown in, Batts, ect…)