The answer to this depends upon the type of spray foam you are asking about. There are two types of spray urethane foam insulation to consider. Each are used in different conditions and to solve different issues in a home.
The first is closed cell with an R value of approximately 6.5 per inch of thickness depending upon brand and application.
The second is open cell with an R value of approximately 3.5 per inch of thickness depending upon brand and application.