What R-value do I need?
This depends on your local code requirements. ICC code allows Icynene spray foam insulation to be installed with less than the prescriptive minimum R-value and still meet the intent of the code.
Compliance based on simulated energy performance requires that a proposed residence (proposed design) be shown to have an annual energy cost that is less than or equal to the annual energy cost of the standard reference design that is based on the minimum prescriptive standard. When the air tightness of the building envelope is factored into the computer calculations, the building will require less R-value than the prescriptive level mandates. Typically the R-value can be reduced by 20-25%, depending on the code version that is being used.
See 2006 / 2009 / 2012 IBC (IECC) Section 506 - Performance-Based Compliance.