I haven't seen Icynene-type foams in a DIY application, but will caution you about a couple things. Assuming you're wanting to spray the rafters??? Is the attic going to be heated/cooled? In my opinion, there's no other reason to do it.
First, blocking off your soffit's ventilation path to the remainder of the attic would cause lots of issues.
Second, a lot of insulators make the dangerous mistake of spraying the stuff up against furnace and water heater flues, and fireplace vents/chimneys. I see it shot into electric boxes all the time, and that is asking for problems.
Third, flammable foam plastics are prohibited in attics that can be accessed for the service of utilities (electrical, furnace, air handlers, plumbing, etc). Many spray foams are very flammable.