US Dept. of Energy Report
US Dept. of Energy published a report showing the different R values of common types of insulation. Notice the efficiency numbers for each type. Closed cell spray foam out performs the others by huge margins! With a 92% ER, just 2″ of closed cell foam is equal to an R value of R-40. It would take 12″ of batt insulation to achieve an R-38. It is also the only one listed that Does Not collect moisture.