Spray foam attic insulation comes in two forms, 1/2 lb open cell and 2 lb closed cell. There are some additional variations for specialists also.
The main difference as suggested in the closed or open cell foam is the structure of the cells within the foam.
The open cell foam has a density which is around 70% open and 30% closed, making it "breathable" and is suited best for timber structures such as roofing or stud work.